To get one of the achievements (and unlocks) you need to use all the combat tactics. Some are more obvious than others, so I am assembled this guide.

Bring them on!
Kill 5 enemies in a single turn
Reward: +2 damage for 2 turn

Get'em from the side
Score 4 flanking shots at non-melee enemies in a single turn
Reward: +1 AP for everyone

Take Cover
Hunker down everyone, with nobody being flanked.
Reward: 20 defense to everyone
(hattip to Reddeyfish)

Too Loud
Throw at least 5 grenades in a single turn.
Reward: enemy reinforcement will span faster [not really a reward, eh?]

Looting Spree
Search for loot at least 3 times in a single turn.
Reward: looting is more effective for 2 turns.
(hattip to Reddeyfish)

Wait it out
Hunker two characters right next to each other
Reward: -5 stress for both (!!!!!!!)

Cover fire
Toggle overwatch next to an overwatching ally
Reward: Characters in overwatch get +20% to hit

Chop and Blast
Hit an enemy with a melee and a ranged attack in a single turn
Reward: The enemy is stunned

Back from the dead
Kill an enemy with a character just revived this turn
Reward: -10 stress for all
(your 4th character rescue can count, so you don't have to let someone fall to to unlock this)
(hattip to Reddeyfish)

Adrenaline Rush
Use a Stimpack to restore health for yourself or someone with 1 HP.
Reward: +1 Action Point for the person healed.