Since you are going to do multiple playthroughs, you do not have to start with Nightmare+ but it is not that hard, once you get used to it. Therefore, you can make the first run, the hard one. Earning the hardest one, achieves the easier ones also.

Take Your Time
Finished the game on the Easy difficulty.

• Self-explanatory.

This is the Way
Finished the game on the Normal difficulty.

• Self-explanatory.

Wasn't THAT Hard
Finished the game on the Hard difficulty.

• Self-explanatory.

Highway to Hell
Finished the game on the Nightmare difficulty.

• Self-explanatory.

Now We're Talkin'
Finished the game on the Nightmare+ difficulty.

• Self-explanatory.

Finishing the missions at Nightmare+ run and doing the side objectives will yield these achievements. The round limit is the same for the nightmare requirement, if there is no side objective of course. However, you can do these achievements at easier difficulties without a problem. All of the missions are main missions. Therefore, you do not have to finish the side missions with side objectives. You can check this guide for a walkthrough: https://www.neoseeker.com/hard-west-2/walkthrough

True Daredevils
Finish "Daredevils" in no more than 8 turns with all objectives completed.

• This is the first mission. It will be a bit hard to complete it but again, it is doable. At least there is no side objective for this one.

First time?
Finish "Fooling The Hangman" in no more than 4 turns with all objectives completed.

• This is the second combat. You need to use dynamites to be able to complete the side quests and even with that, it will be hard. Getting the equipment and clearing the square in one round is tough but doable.

Demolition Expert
Finish "Blasting Out" in no more than 8 turns with all objectives completed.

• This will be easier than the previous two but a challenge none the least. You need to kill 3 enemies at the same time with shadow barrage. Soften with the other characters and finish them off with Gin.

Book of Moses
Finish "O'Kane Farm" in no more than 8 turns with all objectives completed.

• Only side objective is taking the occult grimoire from the house which should not be a problem.

Face Your Fears
Finish "Flynn's Vision" in no more than 10 turns with all objectives completed.

• You need to collect the diary pages only for side objective. Be careful about the doppelgangers and you should be fine.

Looking for Traces
Finish "Frostbitten in Calla Calla" in no more than 10 turns with all objectives completed.

• You need to finish the enemies with skills; so, soften them up and then finish with the skills. I used Bill, Deer and Flynn for this mission.

Close the Rift
Finish "Riftwalkers" in no more than 10 turns with all objectives completed.

• Both side objectives are for collectable items. You should have plenty of time to complete this one.

"Biting" Ben
Finish "A Plague Upon This Land" in no more than 12 rounds with all objectives completed.

• There are 3 side objectives in this mission. One is collectable objective and killing the welcoming committee in first round is pretty simple, especially with explosives. The last one requires you to ricochet your shots for 10 times which you should be careful about it.

Train Job
Finish "3:10 To Heaven" in no more than 1 round with all objectives completed.

• This one is a bit tricky. You need to go to the last cart and the first one at the same time in one turn. It requires a special preparation with cards and items before the mission. Please check the other guide that is provided.

Headhunters
Finish "Wanted: Lady Shrike" in no more than 15 turns with all objectives completed.

• There is no side objective for this one. Just be careful about the last part and keep your distance with Wendigos.

Legendary Robbers
Finish "The Great Train Heist" in no more than 10 turns with all objectives completed.

• There are two side objectives in this mission. First one is not using any healing consumables which makes Lazarus a great choice. Second one is killing the rear guard in two turns. The second one is a bit hard and requires a bit planning ahead.

Final Destination
Finish "One Last Ride" in no more than 12 rounds with all objectives completed

• This is the last combat of the game. There are no side objectives but combat is a bit hard. Take your time and make your way toward the engine. Once you are there, other sections will be closed off. Therefore, you do not have to kill all of the enemies before that area.

Now, where's that train?
Completed all world map side objectives in Chapter 1.

• To be able to receive all of the cards, you need to complete all of the side missions. Since you also need to visit everywhere and quests are straight forward, it should not be a problem. Don't forget to give priority to side quests.
  Beware: According to ZX Speccy Gamer, "At the end of the "Served Cold" mission the game spawns three enemies (Bloodspawn, Revenant Gunman and Revenant Shtgunner). I managed to kill these before the objective "Kill all the enemies before you enter the passenger car" triggers - and finished the mission. That adds two more enemies (Gunman Revenant and Rifleman Revenant). After killing them then completing the mission it was awarded." So, be careful of this mission and not killing all of the enemies before the objective.

Ready for Hell
Completed all world map side objectives in Chapter 3.

• To be able to receive all of the cards, you need to complete all of the side missions. Since you also need to visit everywhere and quests are straight forward, it should not be a problem. Don't forget to give priority to side quests.

Every Nook and Cranny
Visited every location in all Chapters.

• Just move around at the maps and click everything. It is much easier than it sounds. Don't forget to give priority to this with side quests and you will be fine.

Let's Play
Collected every card in the game.

• Regular cards are rewarded for missions, both for main and side ones. You can check the list from here: https://sirusgaming.com/all-poker-card-locations-hard-west-2/
  
• However, joker cards are bit trickier and easily missable. There are two of them at the Hard West 2. You can check this guide for them also: https://gearsrealm.com/how-to-get-joker-cards-hard-west-2/
  
• Black Joker is at Chapter 2's “Wanted: Edward Cullings” mission. You need to choose to play poker with your character and then “I’ll provoke him” >>“Win and provoke Edward” to be able to receive the card.
  
• Red Joker is at Chapter 3's “Wanted: Abraham Burns” mission. To be able to receive the card, you should be able to pick Lazarus for exorcism which is hard, since Lazarus should be a friend. I would recommend doing the card achievement at Lazarus loyalty run.

Maximum loyalty requires 8 points with the same posse member. You can check this guide for choice results list: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2855069087
According to this guide, these are the loyalty points that could be earned (earning one can void another one):
- Cla'lish: 14
- Deer: 13
- Flynn: 12
- Lazarus: 12
- Bill: 11
Since you cannot max out loyalty for all members, you need to do 2 runs at least. I would recommend doing one for Bill, Lazarus and Deer (you can max out Cla'lish also but there is no reason for that). To be able to receive all of the cards, you will be needing Lazarus; so, give priority to him first. Other one can be done for Flynn and Cla'lish. Be careful about maxing one of them early, because it will give you less flexibility at later choices. Check all of the guide contents that is provided before choosing. There is another guide for maxing out all of the characters except Bill. You can try this approach also: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3000035160

Spiritual Bond
Maxed out loyalty with Clalish.

• See the explanation below the chapter headline.

A Witch's Friend
Maxed out loyalty with Flynn.

• See the explanation below the chapter headline.

Now It Is Our Tribe
Maxed out loyalty with Laughing Deer.

• See the explanation below the chapter headline.

Not So Saint Friend
Maxed out loyalty with Lazarus.

• See the explanation below the chapter headline.

A Friend of a Deadman
Maxed out loyalty with Old Man Bill.

• See the explanation below the chapter headline.

Is That All You've Got?
Killed 100 Bandits.

• You will get this one without a problem at your regular playthrough.

Those Freaking Undead
Killed 80 Revenants.

• You will get this one without a problem at your regular playthrough.

Not My Tribe
Killed 50 Shadow Dancers.

• You will get this one without a problem at your regular playthrough.

Demon Hunter
Killed 60 Hellspawns.

• You can get this one at your second playthrough, since there isn't enough enemy for one playthrough. You can try the save-load option (Thanks 76561198001806666 for confirming this).

The Number of The Beast
Killed 666 enemies.

• You can get this at your second playthrough also or you can try the save-load option but I am not sure if it's works or not.

Slow Player
Form sets of Straight with 4 posse at the same time.

• This one is pretty easy. After collecting some cards, you can just make straight hands for all members who is going to fight. "A straight is a hand that contains five cards of sequential rank, not all of the same suit, such as 7♣ 6♠ 5♠ 4♥ 3♥ (a "seven-high straight"). Under high rules, an ace can rank either high (as in A♦ K♣ Q♣ J♦ 10♠, an ace-high straight) or low (as in 5♣ 4♦ 3♥ 2♥ A♠, a five-high straight), but cannot simultaneously rank both high and low (so Q♠ K♠ A♣ 2♥ 3♦ is an ace-high hand)." (From Wikipedia)

Poker Master
Form sets of Royal Flush with 4 posse at the same time.

• This one is a bit harder but easily achievable. "A Royal Flush is a hand consisting of the cards A, K, Q, J, 10, of the same suit." (From Wiktionary)

Passive Rock
Form sets of Full House with 4 posse at the same time.

• One of the easy ones. "A full house, also known as a full boat or a tight or a boat (and originally called a full hand), is a hand that contains three cards of one rank and two cards of another rank, such as 3♣ 3♠ 3♦ 6♣ 6♥" (From Wikipedia)

Only using one weapon type achievements are easily obtainable at side missions, since you have more time and you do not have to complete the side objectives. Save before a side mission combat and try these ones. Of course, it is much easier at easier difficulties but they are achievable at nightmare also. I used the "Village Raiders" mission (Combat is named as "Chase") to be able to obtain these achievements. You can see the location for combat below:



High Noon
Completed a mission using only Handguns.

• Self-explanatory.

Turkey Shoot
Completed a mission using only Rifles.

• Self-explanatory.

Bring a Knife To a Gunfight
Completed a mission using only Melee weapons.

• Self-explanatory.

Full Blast
Completed a mission using only Shotguns.

• Self-explanatory.

These are the resources I used and I would appreciate if you check those links and give them a like:

• General walkthrough for mission achievements: https://www.neoseeker.com/hard-west-2/walkthrough
  
• Poker Card Locations: https://sirusgaming.com/all-poker-card-locations-hard-west-2/
  
• Joker Card Locations: https://gearsrealm.com/how-to-get-joker-cards-hard-west-2/
  
• Loyalty Table: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2855069087
  
• Loyalty Table for all characters except Bill: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3000035160
  
• Poker Hands explanations: https://en.wikipedia.org/wiki/List_of_poker_hands
  
• Royal Flush explanation: https://en.wiktionary.org/wiki/royal_flush
  
• Thanks to 76561198001806666 and ZX Speccy Gamer for feedback.

Thank you for checking my guide. If you have any suggestions or if you see any mistakes (information, typo, etc.), please let me know. Have a nice day!