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