• You are using mods - at least All YAML, so you get access to every weapon and Mech - You have Better Pilots, and Von Biomes
  
• You have read the really good guides on building Mechs for the AI, so they work for you
  
• You have read the weapons guides to understand how they work

• A dedicated Personal Mech - you need one or two Mechs that are just for you to pilot, not for the AI - build whatever suits your play style
  
• At least 3 specialised AI Mechs - 3 to 6 AI only Mechs that specialise in mid to long range weapons - do not use SRM, Machine guns, or Small lasers - use mostly ACs, Gauss, LRMs, PPCs, Large and Medium lasers
  
• You do not have glass cannon Mechs - missions are a war of attrition and you need to survive longer than the enemy - ARMOUR is your best friend
  
• You have matched the pilot to the Mech - battle is about maximising your advantages
  
  Giving AI Mech orders
  Bravo to you if you can manage this, you are better than I am. I don't bother, and this guide does not require you to do it. I just leave them on default - I assume that is just 'follow me'. This guide is based on just letting the AI Mechs follow you around the map. If you have mastered AI Mech orders get them to hold points or focus fire as needed.

Updated: 26 June 2024
I just completed the campaign and imported it into a new career and parked it to come back to later. I then started a new career from ground zero to test the experience and now realised why so many people struggle. The career start is really brutal. e.g. Steiner - a Griffin and 3 crap Mechs. You are paper thin and hit like a wet sock.
My rules for the first missions in a new career:

• Before you do anything else - redesign your Mechs - get max armour and fix the weapons mix
  
• Stick to missions for the major house you start near
  
• You want a minimum of two negotiations points - one for damage insurance - one for salvage - any extras go in salvage
  
• Do missions with limited enemies like defend the base - run all the time and pick your targets - don't worry about the base damage (the enemies are too weak to kill it)
  
• Do objective missions - take your time and isolate and smash individuals or small groups
  
• Assassinate missions can be risky - try to trigger the spawn point and run away - you might be lucky to pick off individuals as they come after you, instead of facing the whole crew at once
  
• Do not do missions for the small factions like Independents or Periphery - crap negotiation points
  
• Do not do destroy the base missions (unless you can get in and out very quickly) - there are infinite waves of enemies - your firepower is low so it takes forever - you will get caught by the next wave - then you face the exit wave that spawns when the extraction point is given
  
• Once you have done all available good missions in the closest conflict zone, go back to the industrial hub where you started - this resets the available missions (travel minimum 3 planets away and back) - the gear available in the industrial zone will reset when you come back

• Salvage a better AI Mech asap - Anything 30T or better - Assassin, Urbie, Jenner, Spider, etc. - save your money to do this - give it to the AI
  
• Get to reputation 2 asap - you get more negotiation points - equals more salvage
  
• Get more better AI Mechs - Same as above - you might see Pheonix Hawk or Blackjack but the salvage cost can be very high
  
• Get cheap weapon upgrades - put tier 1 or 2 Medium lasers into your Mechs when you can afford it

I picked up this idea up from Reddit - someone suggested running all UrbanMechs in your AI lance.
Ever notice Urbies just won't die. Why? Because of the massive armour bonus and can't hit me chance of 15%. The average Urbie has almost as much armour as some 50T Mechs.
How to kill Urbies
Personally, I hate Urbies. They are a real pain early in the game because they won't die and you meet one on every mission. There is no use trading weapons from a distance - there seems to be some Urbie hit bonus and the AC 10 hits way too often - so just charge them with your 50T Mech and punch them to death. They will usually fire their MG, or small lasers up close (hopefully not the AC 10).

The best way to win a mission is to deploy the right Mechs and negotiate correctly
Essentially there are a limited number of missions
Regardless of the name they give them, I class them by what you need to do.

• Kill the targets
  
• Destroy the base
  
• Defend the base
  
• Go to all the locations and find stuff
  
• Beach head - satellite dishes, artillery and Mechs
  
• Multi turn missions

• Early on, when your lance is weaker - take damage insurance
  
• Destroy the base - take airstrike
  
• Beach head - take airstrike
  
• Go to all the locations and find stuff - go salvage or money
  
• Kill the targets - go salvage or money
  
• Defend the base - go salvage or money

If you are finding a certain type of mission really tough, deploy Mechs over the tonnage limit. Just a bit more armour or a few extra weapons can tip the balance your way. You can cancel out the over ton penalty by adding more points to salvage.
Read the section below on multi turn missions.

If you have spent time managing your lance correctly you will have an advantage over the enemy. Your objective is to build Mechs that outclass the enemy in every way for their weight class. You should aim to have the best balance between speed, firepower and heat.

The problem early in the game is that learning how to fight is difficult. This is a hard game to learn, so give yourself a break and save often, and don't be scared to restart a mission if it all goes to crap.

• Don't just charge up to the objectives - you will get ambushed
  
• Don't get isolated away from your lance - you will get surrounded
  
• Don't get your lance caught in open ground surrounded by enemies - use terrain cover
  
• Draw the enemy into your kill box, don't walk into theirs
  
• Learn what or who to destroy first
  
• Speed is important..........and sometimes it is not

On key board, B opens the batttlegrid map. Before you go anywhere, open the battlegrid and look at the locations and terrain to decide the route you will take. The other benefit is you will often spot the next wave of attackers coming for you on the larger battlegrid better than the tactical map. Especially if there is an early wave of flyers coming at you. Caution: sometimes you will be attacked immediately on landing, so clear the landing zone first. Obviously, don't sit looking at the battlegrid if artillery is active, just run. Pro tip: As soon as you hear the "weep weep" noise of flyers approaching open the battlegrid - you will see exactly where they are and can turn to meet them - drop the battlegrid and shoot
Never take the most direct path to the nearest objective. Why? Because the Devs designed the map to suck you in and placed the enemies where they can hurt you the most. This is how all games work. I have 2000 hours in BattleTech and 500 hours in MW5 and never take the obvious route to the objective. Sure, you have to walk further, but then you might not die.

What route do I take?
The strategy here is to avoid open ground, approach from cover, avoid the obvious routes and avoid choke points designed into the map.

Outflanking the Devs - check the corners
As a general rule maps are square or rectangular. I always walk along the edge of the map to the next corner and get to the objective from the 'wrong' side where the Devs didn't place the turrets or Mech and Tank spawn points. This can be very important on multi turn missions (see below).
Sure, on some maps there is only one usable path because cliffs or valleys block you. Ok, think out of the box. Walk up to that cliff, the one that is designed to block you and force you through a choke point 900m away, and jump off the cliff. If your AI Mechs don't follow, take manual control and jump then off yourself. Now you have outflanked the Devs and avoided their trap.

On your way to an objective
On your way to an objective you will get attacked by harasser forces. These are placed at likely approaches, or are random, to wear you down before you get to the main objectives. There will be flyers, pop-up turrets, tanks and Mechs. Keep you eyes and ears open and watch your tac display for the red dots to show up. Minimise their impact by hitting them hard and fast. Pop up the battlegrid frequently, it shows enemies at a better distance than the tac display.

Flyers: your best defence is LL x 2 - this will kill most VTOLs at range. Igors are much tougher.
Turrets: pop up close to you - ML x 2 or 4 will kill most of them
Tanks: LRM from a distance (if you are lucky) - Chase them down, laser them to death or run over them and they get stomped.
Mechs: They might be only one or two so surround and kill

When you get near an objective your basic strategy is to break up massed forces into smaller waves and pick them apart on your terms, not theirs.
As you get near the objective you will cross a threshold or trigger point that will make the enemy AI do something. Hidden enemies such as Mechs, flyers or tanks will spawn or artillery will start to fire. Be prepared and don't just blunder into traps. Look at the terrain around the objective and look for the approach with the most cover. It is better to walk 1000m around the back to approach from cover, instead of walking in the front door to get murdered. You always want to have a fallback point with some cover you can run to if you get in trouble.
Pop and drop
There are a lot of objectives 'just over the ridge' from where you are. When you crest the ridge, bam!, 4 Mechs spawn and unload on your lance. Stop just below the ridge and creep up to the edge and pop your head over - the second they spawn punch it into reverse and drop back over the ridge.
Trip and run
If you are on flat ground get behind a hill, run out towards the objective to trip the enemy spawn then run away back behind the hill. Have a second hill or valley to run to if they all come for you at once.
Shoot and wait
Can be used anytime you have line of sight and a weapon in range, such as a LL or ER LL. Get to about 1000m stop and shoot, then wait. Make sure you do enough damage to the structure. Often after a few seconds a wave of flyers or tanks, or a few Mechs will come at you from somewhere (even behind you). Drop behind cover and pick them apart one by one. Don't get caught in a hammer and anvil situation. If attacked in the rear, turn and run at whatever spawned behind you and kill it first, then turn back to the enemies coming from the objective. You will put distance between you and the Mechs coming from objective giving you time to deal with one threat at a time and clear your rear arc.
Ridge line ambush
My fave tactic for tanks and light Mechs is to sit just below a ridge line and wait for them to cross over the top and melt them before they even get a shot off. Target the victim and watch the red distance count down as the come to the other side of the ridge so you know exactly when and where they will appear.
Next steps
Now that you have triggered the enemy to spawn, and if you are in cover and not under direct attack, make an attack plan. Cycle through the enemies using the target key to identify them. Open the battlegrid to look around. Stay still and wait for any flyers or tanks, or scouting Mechs and take them out first. If you are charged by the whole garrison at once, consider running to your fallback position.

Often one or two light or medium Mechs will break from the group and chase you. Perfect - they will run into your kill box and you can 4 on 1 or 4 on 2 kill them without facing fire from their big buddies still behind the ridge or hill. You can pop a UAV so LRMs can see and lock on from cover.

Don't be scared to run away. Some maps and objectives are designed to punish you unfairly. Just run away and refuse to fight on their prepared ground. Learn to fight in reverse!

Retrograde - or fighting in reverse
Running backwards and shooting at a Mech closing in on you is not easy, but you need to learn this. Why? Because they are trying to get their big gun in range to blow your head off (Hunchback + AC20) and often you only have Large or Medium lasers or AC 5 or 10 or PPCs and you want to keep some distance. Also speed puts off their aim. Aim at their legs to slow then down. Remember steering left and right is opposite in reverse and don't forget the 'centre my torso' key (C on keyboard) can be handy. To go forward again hit X (keyboard) to stop then run forward to engage when you have the advantage.
Enemy Mech behaviour
There are basically three types - brawlers that just charge you - guys that stand their ground - snipers / retrograde LRM boats

Chargers - they want to melee punch you or AC 20 you in the guts - fight them in reverse and target their legs to stop them closing to melee range - jump if it helps you evade melee - then take out the other leg - e.g. Centurions, Hunchbacks, Orions

Standing their ground - run full speed left or right across their front and don't get too close - speed ruins their aim - circle them either taking out legs or arms to slow and weaken them e.g. big Mechs like Marauder, Banshee, Cataphract

Snipers - charge them full speed - throw in some random left or right turns to put their aim off as you close the distance - go for the legs to stop them backing away from you e.g. Blackjack, Rifleman, Jager, Catapult

This is a special case.
I almost always charge the artillery and take it out asap. You need a Mech speed of above 60. 80 is really good for this. Look for the red marked location on your heads up display and remember where you saw them because they only show for a few seconds are they fire and can be really hard to find. If in doubt, check the corners of the map. This is where they often hide.

You will eventually learn how to avoid incoming artillery. Go max speed and dodge hard left or right to get outside the circles when they show on the map ahead of you. Sometimes you get boxed in and need to slam on the brakes then run again. Do not worry about your lance being hit, they will be too slow and out of the hit zone as you get ahead of them.

Next, if there is a central base, DO Not go within 1000m of this base (marked by a big star on the battlegrid). Why? Because enemies will spawn at this base and chase you. You do not want to have Mechs chasing you at the same time you are being hunted by artillery.

Take out all the satellite dishes near the edges of the map first and save the big one at the base until last. This will call in the support allies. Then mop up. I have even walked 4000m back around the other side of the map to avoid tripping the alarm at the main base. Trust me this can be a life or death mistake on 90+ difficulty missions. You do not want to trigger a lance of Atlas, Nightstar, Cyclops, Marauder strength at the wrong time, and face artillery fire.

If you have experienced some odd or interesting scenarios with artillery and need more advice go to the spoilers section at the end of the guide.

I always use Better Pilots mod
I like the RPG aspect of customising your own pilots. I also love the stupid comments and silly nicknames they use - seriously, who would call themselves "PainGod". The choice of setting is important here. I chose any level of pilot available anytime setting, not the level of pilots scales to your reputation. This is because training and developing a pilot takes a very loooong time. It is no fun to only get pilots with a mediocre max number of skills points and spend hours developing them only to dismiss them because someone 5 times better just showed up. Try to hire pilots above 60 points as early as possible.

Aside from enhancing the enjoyment of the game, you get great (semi-customisable) skills.
6 skills. Better damage with each of the three weapon types. Damage reduction, weapon evasion and better heat management.

Always prioritise Damage reduction, Weapon evasion as early as possible. Why? These give you the best survivabilty bonus. Less damage or avoid a weapon hit - equals less total damage. Look for at least 10 or better when hiring pilots. My best pilots all have about 14 or better in these. Online advice is Skill level 10 is sort of a soft cap where you hit the most benefit quickly and it gets less per point above 10. If you have someone specialised in Ballistics, or Lasers or Missiles, put them in the correct Mech to combine more damage and damage reduction. A definite win win, for you.
A few things you can try

• Damage sponge - a Mech with a lots of armour that runs up front to draw fire - protects the rest of your Mechs, letting them smash from the back
  
• Laser vomit boat - (see the Mech guides) - this is Mech with 8 or 9 Medium lasers and good heat management - people recommend the Hunchback-4P, or Blackjack -1X
  
• My personal fave - Marauder II-4A - 100T Mech with hardened armour and max lasers - 1200 points of armour and 70+ alpha strike - it does both
  
• Too much aggro - if you are taking all the fire - run away - run back through your lance - one of your Mechs will take the aggro - run around the side (behind cover if available) and do a flank attack

Always getting your arms blown off?
Try adding 'Modular Armour' or Modular Ferro Armour' to the arm. Gives extra armour to the location it is mounted - but still costs some weight - you can have a Heavy Mech with Assault Mech arms
Add a battle fist - gives 10 extra armour to the arm - only weighs 0.5 tons

Dealing with 'Death from Above' - jump attacks
I really get annoyed when some little crap Mech jumps on my head.
Try this tactic - RTS - Run Turn Shoot
RUN - If you see a jumper going above you for a 'Death from Above' run max speed towards them as soon as they jump - you should run under them and they should fly over you - a jump is easier to adjust left or right or forward, but not backwards
TURN - hit X to stop and turn quickly - apply some speed to get around
SHOOT the little bastard or punch them as they land

These are where you can make the most money. Use the mission assessment ideas above and decide what you think you need. Remember you only negotiate once, so think about mission 2 and 3 up front.
Special things to note:

• The extra payout is for all three missions - this can mean earning 3 x the money - so go big
  
• You can't repair Mechs between missions - but you can swap for another fresh Mech without any damage
  
• Think about the order of the missions - if it starts with Beachhead - take a fast Mech
  
• Swap out a damaged Mech or bring in a better defence Mech - for Defend the base missions
  
• Almost always take one Airstrike - you might need a quick base destruction or kill a group of Mechs

Swapping a Mech that is too damaged to continue - I discovered it by accident and here is how to exploit it.
In between each mission check the armour loss in each Mech to decide if you need to swap it. This allows you to make up for bad luck it one Mech go pounded. It allows you to deliberately get one Mech pounded as a damage sponge in one mission to preserve your other Mechs for later missions and swap the sponge out. Over the course of three missions you will have spread the damage across 6 Mechs, not 4 and without any single Mech losing an arm or leg. You can take damage insurance to pay for this and maximise your earnings.

Warning - spoiler text will appear if you mouse over the black sections below

Artillery alternate scenarios

Hidden Artillery
Some missions start with the artillery hidden. This happens more often as the difficulty gets higher. The mission will start and you expect artillery to be pounding you, but nothing happens. You think sweet, no artillery, this is easy. They are hiding and will only start once you trigger the central base (another reason to avoid it early). Just run to each corner and find them. They will turn red as you approach so kill them. Very occasionally they are near the middle of the map. The funniest part is Rayana will tell you "look out boss hidden artillery, go kill them", then 5 sec later "hidden artillery destroyed", because you already killed them 10 minutes ago.

Additional Artillery type one
Ok, you start with active artillery and you seek and kill them. You go to the base and surprise more artillery hit you just as the big Mech lance spawns. To avoid this I always sweep all four corners of the map before going to the main base and laugh as Rayana tells me the sensor sweep missed additional artillery and to watch out.

Additional Artillery type two
Just as you finish the mission, surprise more artillery. This has only happened to me a couple of times, so I am not sure if you can remove this threat by doing a proper artillery sweep prior to the last objective. In any case I did not hang around because a nib lance was landing at the same time, so ran for the jump ship.