As an intro I would like to say that I haven´t done much research , I just fooled around with a few characters that I liked and ended up with this team . I didn´t know most of the mini bosses , but I killed them all while playing through the grand slam run, including the Shambler and the Collector. Of the Lair bosses I killed the Dreaming General , the Harvest Child and the Librarian , but I´d suggest to just go for the Dreaming General as it is a free kill with this team really.

Position 4 : Plague Doctor - Alchemist
Mainly a damage dealer , but also a healer/cleanser in crucial times. Very rarely used for her cc with Blinding Gas , for example when a big enemy is ready to swing a big aoe from the back line, or a heavy hit from a boss without enough taunts/shields currently on my team. Remember , the best cc is killing the enemy !

Skills

• Noxious Blast
  
• BlindingGas
  
• Battlefield Medicine
  
• Plague Grenade
  
• Indiscriminate Science

A pretty straightforward character , you start by nuking the backline with Plague Grenade and then moving on to the first 2 enemy positions with Noxious Blast. I rarely had to use the Battlefield Medicine, and I mostly used it when a character had stacked a huge amount of dots (even before they get to healing range). The Indiscriminate Science becomes way more useful after the upgrade, but all in all a really powerfull skill with some nice unexpected heal bursts.

Position 3 : Occultist - Warlock
This is our heavy hitter ! After some hours of playtime I figured that I didn´t like both of my backliners to do dot damage , so I tried the Occultist out and he worked like a charm.

Skills

• Abyssal Artillery
  
• Daemon´s Pull
  
• Binding Shadows
  
• The Burning Stars
  
• Chaotic Offering

At the beginning of the fight I nuke the enemy backline together with the PD, using Abyssal Artillery.
When they are dead (or ready to die on their turn from the blight damage) I check the health of the enemy front 2 positions and my Occultist´s charges. If I have 2 charges I nuke them with a Burning Star (it ignores shields btw). If he has <2 charges and there is an enemy that is ready to die or will be killed after all my team plays this turn I use Binding Shadows. In all the other scenarios (I have 2 healthy front line enemies , he has 0 or 1 charges, the fight is going to take more than just another 1-2 turns etc) I use Chaotic Offering.
-Keep in mind that you store up to three charges on you , so even if you have 1 charge and you use the skill you will not lose any charge if you use The Burning Stars (or very rarely a buffed Abyssal Artillery) next turn.
-The Daemon´s Pull slot is basically his utility slot for corpse clearing and can be switched, for example for the Vulnerability Hex against bosses with dodge (like the Mountain´s 4th boss) .

Position 2: Vestal - Chaplain
Our main support , via hots, guarding, shielding and a bit of stress removal.

Skills

• Hand of Light
  
• Mace Bash
  
• Sanctuary
  
• Divine Comfort
  
• Mantra

She has in my opinion one of the most broken skills in the game , namely the Divine Comfort. It is the first skill I always level up in the first inn , and the first skill I use in 99% of the fights with her. The only reason I wouldn´t cast it on turn one is if there is a really pesky enemy on the enemy position 3 that needs a couple of damage to get into lethal range from PD´s blight. Her marks are mainly used for the Mantra self heal and stress reduction , which becomes even more usefull and her party stress removal after the upgrade. The Hand of Light skill is the filler and a pretty commonly used skill, so it´s worth the upgrade since you are giving shields to positions 1 and 3. The rest is just guarding your squishy backline and Bashing the enemies (which also ignores dodges and shields when boosted with 2 marks).
-Instead of using 2 marks on a Mace Bash consider filling in with Hand of Light in order to use a 3-mark-boosted Mantra , which cancels out a ton of enemy damage and stress attacks.
-If you need a slot for some utility (like Consecration of Fortitude against a known single target heavy hitter, or Illumination to remove enemy dodge tokens) I would remove the Mace Bash.

Position 1: Flagellant - Scourge.
Our tank, backline dotter, front line nuker , self and team healer, the Flagellant !

Skills

• Punish
  
• Deathless
  
• Acid Rain
  
• More! MORE!
  
• Sepsis

The Flagellant´s playstyle is pretty simple, you start by casting More!MORE! , then something else , then back to More!MORE! . In the case of super pesky enemy backliners that need a bit more dmg in order to die turn 1 (and the rest of my team is healthy) I may cast Acid Rain turn 1 and then start the More!MORE! cycle , but it´s kinda rare.
-Keep in mind that your self-harming skills give tokens, so not only you dish out damage but you heal more at the same time.
-If the enemy team is hitting hard and your Flagellant is <50% while having a few tokens, a well placed Deathless means that he goes back to full hp and you heal another teammate at the same time. If he is <50% and More!MORE! is castable it´s often better to just use More!MORE! to heal him back up the next turn.
-When there are beefy enemies/bosses, Sepsis (especially an upgraded one) becomes a nice atomic bomb, so keep that in mind. I found it useful to nuke an enemy to a death´s door although it removes the blight from him (when he isn´t marked), because a fast team will finish the enemy next turn before he gets to do anything (more on the speed part on our next and final member). All in all a really powerfull skill, but you have to do a bit of counting in order to see what is better, Sepsis or another stack of blight.

Position Pet : Reanimated Rabbit
An often overlooked slot, but crucial for this comp.




Skills

• Speed Boost
  
• Cuteness
  
• Fluffiness
  
• Health Boost
  
• Smoll

I found the rabbit to be very important in the pet slot , and I used it for all the acts. The speed boost is crucial to bursting down the enemy backline, because you get to stack them with damage and dots before they play , which means they can easily die at the start of their turn before they get to do anything. The added bonus of health points is also super usefull because 2 of our 4 team members have reduced max hit points due to their paths.

Although I gave specific tips for every character and I believe you already got the basic idea of the focus of my team , it wouldn´t hurt to lay down a gameplan with some extra tips.

Target Focus
I don´t know if you already got it, but I hate the enemy last 2 positions. It´s where the most annoying enemies usually are, but the good news is that they are also usually the most fragile.At the beginning of the fight we hit them hard and fast , and after they are dead or dying from blight we move on to the next more annoying / dangerous enemies, on positions 1 and 2.At that point our Occultist will either have 2 charges or will charge them up with Chaotic Offering,so check if you can oneshot an enemy with a Burning Star,else use it on the beefier ones to burst them down quickly and through their shields.
-Enemies in the backlines often have dodge tokens.Don´t let them confuse you, just stick to the plan. You remove tokens from two enemies with each of our backline attacks, plus the attacks will hit 50%/25% of the time anyway.
-If there is a big guy in the enemy 4th position, meaning the aoe attacks only hit him, I go straight for a Chaotic Offering with my Occultist, and then either kill the squishier targets in the front, or focus the big guy in the back. Especially against the Cannon don´t waste any time, load up those Burning Stars asap.
-If your characters get hit by something and they lose some hp or get a couple of dots dont panic, if the sum of the tick is 4-5 dmg you heal the 3 of it from Vestal´s hot anyway.Keep in mind that you heal while on the road too, so killing the enemy team is more important than having the PD removing dots left and right.Guard the squishies with Vestal, spam taunt with Flagellant and the team will be topped up in no time.

Skill Level Up
The first two skills I level up are Vestal´s Divine Comfort and Flagellant´s More!MORE!, since those two skills take care of most of the healing I need to do in most of each act.
Next I focus on damage output, meaning Occultist´s Abyssal Artillery and PD´s Plague Grenade.After that its a balance between support boost from Vestal, and more dmg upgrades for our main dpsers.I tend to pick Vestal´s Hand of Light early on since it´s her bread and butter damage skill and a nice defensive boost at the same time.
Flagellant´s kit is super strong anyway, so I don´t prioritize him. If I have a lot of points to spend I level up his dmg if it´s early and Deathless if I´m moving towards the Mountain.

Trinket Build
Not much to say here, I used dmg boosting trinkets for my backline, team buff/tanking trinkets on my Vestal, and tanking/movement resistance/team buffing trinkets on my Flagellant.Until they all get usefull items, I use the survival items on my squishes.
-The Plague Doctor needs any extra dot/blight duration for her dps boost, whereas the Occultist needs raw dmg boost/critical chance etc.
-Keep in mind that The Foetor region has all the broken items for the Plague Doctor.

Managing the weaknesses
Stress and shuffling can be dangerous for any team. Thankfully the occultist does well even if pulled to the second position, and we have two characters with double movement (Occultist can move 2 back and the Flagellant 2 to the front).
-The Flagellant is super weak if pushed away from the second position, and he is tanking most of the hits that can move anyway,so I give him a move resist buff with inn-usable items and trinkets, while avoiding move resist reducing trinkets.
Stress on the other hard is not so easy to counter with this team,so I try to have a character with 4 Laudanum on him at all times.Can´t say I had a big problem though, mainly because the enemy stress-dealers (usually on the backline) die so fast. I think I had 3-4 toxic meltdowns on my Flagellant for the whole 5 acts, and maybe one or two for the rest of the team, on the first Mountain Boss.After a while Vestal keeps not only herself stress-free, but picks a few stress points off of the rest of the team too.

As I said before, I didn´t do any big research before I dove into the later acts of the game, I just played around and found out.I´m pretty sure that there are in-depth guides for every section of the game,but this is what I found out that helped me progress until I reached the grand slam!

Unlocks
-If your travels seem to be too difficult, keep unlocking stuff in the Altar of Hope,and soon you will see that with a bit of boost here and there you can overcome previous difficulties.The Journey, Resourcefulness and Companionship (up until the Rabbit) should be your top priorities,plus of course the passive hero bonuses of your main 4 in The Living City.
-It´s hard to say in what order to unlock the items in The Working Fields, because they all help in their own way , but I would suggest unlocking all the Food Gear for the Stargecoach in order to get the most boost from the Rabbit, plus the free food and whiskey help from the first to the last inn.
-I found the Inn Items to also be extremely important, because of all the relationship boosting items that you unlock. More on relationships (in-game only sadly 😭) below.

Stagecoach Items and your Baubles
There are 5 Stagecoach Food Gear items available, so just use 4 of them to get the most out of your Pet.The Food Barrels and Griddle are a priority, since they combo so well together and you wont have an inventory full of food right from the start.
I never used Baubles early on for Trinkets, I saved up for Food Gear until I got 4 of them.

Navigation through each act and relationships
-My main priority while navigating through an act is to gather as many Mastery Points as possible, simple as that. Many of the non-combat locations affect relationships, which can be very annoying early on if you get a few bad deals.I also liked the road battles more than paths with Hazardous/Rough/Loathing, because I didn´t have a problem with them and they are a good way to actually reduce stress and build up relationships if you are doing well enough.
-If someone has a really bad quirk / sickness I aim for the Hospital. Whatever is affecting Max HP or Damage has to go asap, and then I try to mitigate whatever affects Relationships.
-At first I really hated the relationship part, mainly because I didn´t pay much attention to it and as a result ended up with everyone hating each other, but also because I didn´t have enough relationship-boosting Inn items to affect it. Think of relations as a minigame , you try to keep everyone at at least 10/20, so in the various locations try to balance the negative/positive outcomes to keep everyone neutral,and then boost them all up in the inn. Even from the first inn I use every relationship boost item I can find, and I save the Whiskeys up to even things out later in the game.
-I´ve mentioned it before, but from the Lair Bosses (in order to get a Trophy) I would always choose the Dreaming General in The Tangle. His mechanic requires that you hit the 4th position with any attack (well , 3 of your 4 heroes have an attack to hit both the boss and his Root), and the heavy attack is telegraphed which gives you a turn to protect the squishes with Vestal´s Sanctuary. Even as your first region he is an easy kill, so don´t hesitate to pick up his Trophie and be done with it.Don´t be too hasty on equipping it though ;)

Combat Items
The Big 3 that I always try to keep on my team are :

• A Glimmer of Hope
  
• Laudanum
  
• Healing Salve

All self explenatory, I like my flame close to 100 at all times, my Stress near 0 and my health topped up.
-For the extra slot I used Powders a lot , depending on the region , because they help my tanks with both damage and stress mitigation. Then you have some really strong attack items which I used whenever I found the opportunity. I firmly believe I didn´t lose any hero on the third Mountain Boss because I had 2 Blood items on my Occultist, so I nuked the second phase before he got any chance to kill anyone on my team.