I spent over 10 hours attempting to complete these last three achievements. In that time, I found out they are actually pretty easy. You just need the right strategy.

Completion Time: 1.5 Hours

This is pretty straight forward, and can be done with the starting unlock characters. If you happen to have the Stun Module unlocked though, it will be a bit easier. For that, choose characters with a revolver.



Mycroft Valentine:

• Ground Slam: For "That’s a Big Thump" Achievement
  
• Search Old Stashes: Helps find more grenades in shelter.

Michael Kipps:

• Tactical Order: Grants team members an extra action point.
  
• Revolver: Stun Module

Hector Turner:

• Your third choice can be whoever you want. Hector is a great character, with a revolver.
  
• Revolver: Stun Module

Support Unit:

• Stun Grenade
  
• Intimidate: Cause ranged enemies to push forward.

Easy: Not enough reinforcements.
Normal: Perfect balance, choose this one.
Hard: To many high health enemies.

You will be completing these in Chapter 1. By going further, the enemies will have too much health.

Play until the second to last section, to allow yourself time to collect resources and level up.

You don't want to continue to the bridge, because of how open it is, with spawns on both sides.

• Search every lootable.
  
• Save grenades and fusion cores.
  
• Add the Stun Module to your revolvers if available.
  
• Upgrade your character abilities according to the previous section.

• Focus on Grenade Mastery first, than Hotter Fusion Cores.
  On level 2, Grenade Mastery can stun enemies, which is crucial. The Fusion Cores damage also increases more per upgrade, so it can be done last.

Once on the second to last level, go to the end and start opening the bunker door. This will increase the enemy reinforcements.

Next, find a room or hallway you can barricade yourself in. My extraction was actually in a hallway, so I got lucky. Ideally, the space leading to you should be 3 wide. This will force the melee units to stack up next to each other.

It may be tempting to use overwatch, but just let the enemies come on in. Kill the ones that get too close and stun the others. Use your Stun Module and the Support Unit first, to save your grenades. Your Support Unit can use Intimidate to force ranged enemies to advance forward.

As the enemies stack up, shoot the ones with a lot of health. You want to make sure the health is below the base damage of your grenades and fusion cores.

Once you have a good collection of enemies, SAVE AND EXIT TO MENU. You can use this save for all three achievements.

Load in the game, then get one of the achievements. DO NOT END THE TURN. After you receive the pop-up, ALT+F4 (To Exit Game). By exiting this way, it won't save your turn and the enemies will still be there.

Just repeat the process until you get all three and you are good to go!