This is just a short guide to help people get 100% achievement completion.

Figured I'd also throw in a few tips for the actual walkthrough.

• Pick a squad composition that works for you, then stick with it. Some missions require you to use certain characters, but for the most part, you can field whatever comp suits you best.
  
• For my part, I actually used the first three characters the most: Linda as close-range/melee, Samuel as a sniper, main guy as main guy.
  
• As for later characters, Subject 13 is a beast in close range with proper equipment. Gina Nia's paralyze ability is straight-up broken. The other characters (Josh, Dr. Schaffer, Jenna) I found to be just 'meh'.
  
• You'll find enough stimpacks to cap out all your characters at 5 stims each, so there's no need to be stingy with them.
  
• Same goes for consumables. Use your medkits, shield batteries and grenades liberally. You'll have plenty left over by the end of the game.
  
• Explore each level to make sure you've found all the lootable containers (use ALT to highlight all interactable objects).

You will earn all these achievements simply for finishing the game, no special effort required.


Linda Carter
Linda Carter joins your team.


Earned after rescuing the first new team member, right at the start of the game.


What the... ?
Kill Johannsen, or what's left of him.


Kill your first enemy. He appears right after you rescue Linda.


Samuel Lankford
Samuel Lankford joins your team.


Earned after rescuing the second team member, in the first mission after reaching the homebase.


Josh Connor
Josh Connor joins your team.


Team member number four, rescued as part of the story like all the others.


Dr. Schaffer
Dr. Schaffer joins your team.


Subject 13
Subject 13 joins your team.


Full House
Have five characters join your team.


You will find team members number five and six together in the lab during the rescue mission.


Jenna Wallace
Jenna Wallace joins your team.


Gina Nia
Gina Nia joins your team.


Lucky Eight
Have all characters join your team.


The final two team members are rescued during the same two-part mission roughly halfway through the game.


Are we there yet?
Complete the game's main story.


Earned right after you kill the boss in the final level. He's barely stronger than the armored elites you've been fighting for the past few levels, so it's not a particularly difficult fight. Feel free to ignore his minions. The level will end as soon as the boss is dead, so there's no need to mop them up.

Scavenger
Loot and empty 10 containers.


Master Scavenger
Loot and empty 50 containers.


Containers are boxes, lockers, etc scattered around every level, containing ammo, weapons, medkits and such. You should easily reach the required amount of containers pretty early on.


Loot Hoarder
Have the team storage at full capacity.


Your homebase has a stash that can hold up to 50 items. Just dump all the stuff you find during missions inside it. It will fill up pretty quickly.


Full Bags.
Have the backpack at full capacity with at least three characters.


Each character can carry up to 8 items (weapons, medkits, ammo, etc). Samuel can carry 12 thanks to his passive ability. There's tons of stuff to loot in every level and you'll likely be struggling to pick it all up, so this achievement should come quite naturally.


Conservationist
Recycle loot to gain 1000 energy.


You can scrap any weapons and other equipment you don't need for 'energy' that you can use in your homebase to buy ammo, medkits and grenades between missions. You should reach the required amount in no time.


Mutant Hunter
Kill 20 enemies.


Pest Control
Kill 100 enemies.


Stop the Invasion
Kill 500 enemies.


This counts any enemies killed (mutants, turrets, etc). There's only about 400 enemies in the whole game, so you will need to start a second playthrough to reach 500 kills. I recommend playing just until you get your third member (Samuel). Right afterwards, your base will be attacked by a group of weak enemies. Wipe them out, then restart the level and repeat until you've hit 500 kills.


Sidequest Bob
Complete an optional mission.


Sidequest Hero
Complete all optional missions in one single play-through.


Optional missions are marked on the map with yellow dots (main quests are green). There's 7 optional missions in total. You can do them at any point after they become available, but be sure to finish them all before the final mission. They're mostly just short scavenging runs that can provide you with some unique and powerful equipment, so you'll want to do them anyway.

Double Kill
Kill two enemies in one turn with the same character.


Self-explanatory. Each character has two action points per turn, so just use a 1-point action to take out two enemies back-to-back with any character. Or just use a grenade.


Hot Shot
Kill an enemy at full health with a single ranged attack.


Just one-shot any enemy. Easy enough with a high-damage weapon, such as a sniper rifle or railgun. Samuel's Headshot ability (guaranteed crit) can also help.


Killing Spree
Kill four or more enemies in one turn with all characters.


You just need to score four kills during your turn by any means necessary. Easier to do when you have a full four-member squad. Even easier to do with grenades.


Last Man Standing
Survive a fight with only one of at least three characters fit for action.


Whenever a squad member's health reaches zero, they are 'knocked out' for the duration of the current battle. Just have all members except one get knocked out, then kill any remaining enemies with the sole survivor.

Attentive Audience
Carefully listen to conversations without interrupting them.


You should earn this by not skipping through the dialogue bubbles. You simply need to do it for a certain amount of conversations, not for the whole game. However, I got it on the very first level, after having the ship AI extend a bridge to access the southern part of the map. Possibly a bug.


The cake is a lie!
Find the hidden cake.


Once you've assembled all eight team members, you will be tasked with going to Josh's apartment as part of the main story. While you're there, you'll need to kill a security bot that suddenly appears from a hidden passage behind a wall.

DO NOT HEAD TO THE ROOM THAT OPENS UP AFTER KILLING THE ROBOT

This will end the level and you will be unable to return to the apartment, meaning you'll have to restart the entire game if you want to get this achievement.

Instead, enter the secret passage from where the robot appeared. This will activate two red tiles in the bathroom. Head to the bathroom and stand on the two tiles. You will be teleported to a hidden room that contains various Portal references and you'll get the achievement.

Congrats, you've earned all the achievements in Halfway!

Hope this guide helped.