Here is the full list of all 18 ZAMB Endless Extermination achievements.
Finish a stage with critical health level
Finish a stage when one character is down
Buy all upgrades of a single power
Buy all upgrades of a single turret
Buy all upgrades to all the powers
Buy all upgrades to all the turrets
Kill your partner
Respawn your partner
Complete a level without building turrets
Complete a level without buying powers
Create at least one PWNAGE of each kind during a stage
Create a PWNAGE of your choice with a barrel
Level up during a stage
Charge Cye's Boost bar completely and use it
Reach the limit of planted traps in a stage
Charge Chrome's Boost bar completely and use it
Reach the limit of built turrets in a stage
Beat all levels