Here is the full list of all 36 Sniper Elite: Zombie Army 2 (DE) achievements.
Kill 20 Snipers in co-op
Kill 5000 enemies
Defeat the Final Boss
Defeat 20 Summoners
Kill 2000 enemy grunts in co-op
Kill 20 Elites in co-op
Kill 100 skeletons with explosives
Kill a suicide grunt by hitting his grenade
Defeat 500 enemies with headshots
Get a 5-for-1 kill with any sniper rifle
Defeat 10 Fire Demons
Defeat 100 ignited zombies
Kill 1000 enemies
Kill 20 enemies as they revive
Successfully complete Chapter 5 with 4 players
Successfully complete All Chapters 4 players on Elite Difficulty
Kill any 100 enemy with the shotgun
Collect all Gold Bars
Find and shoot all the bottles of blood
Kill the trapped Elite
Successfully complete Chapter 4 with 4 players
Loot an Elite's corpse
Kill 500 enemy grunts
Kick 50 enemies down in co-op
Defeat a Fire Demon
Get at least 15 for 1 explosive kill in 4 player co-op
Get at least 20 for 1 explosive kill in 4 player co-op
Successfully complete Chapter 3 with 4 players
Pick up a phone
Get at least 10 for 1 explosive kill in 4 player co-op
Successfully complete Chapter 2 with 4 players
Successfully complete Chapter 1 with 4 players
Become possessed
Pick up the Preacher
Pick up a shotgun
Own 2 Nazi Zombie Army games