Here is the full list of all 26 Sniper Elite: Nazi Zombie Army achievements.
Kill 5000 enemies
Kill 100 skeletons with explosives
Kill 20 Elites in co-op
Kill a suicide grunt by hitting his grenade
Kill 2000 enemy grunts in co-op
Kill 1000 enemies
Get at least 20 for 1 explosive kill in 4 player co-op
Successfully complete Chapter 5 with 4 players
Successfully complete All Chapters 4 players on Elite Difficulty
Kill the Occult General
Collect all Gold Bars
Find and shoot all the bottles of blood
Kill 20 enemies as they revive
Kill 500 enemy grunts
Successfully complete Chapter 4 with 4 players
Successfully complete Chapter 3 with 4 players
Kill 20 Snipers in co-op
Successfully complete Chapter 2 with 4 players
Kick 50 enemies down in co-op
Kill any 100 enemy with the shotgun
Get at least 10 for 1 explosive kill in 4 player co-op
Get at least 15 for 1 explosive kill in 4 player co-op
Successfully complete Chapter 1 with 4 players
Pick up the Preacher
Pick up a shotgun
Own 2 Nazi Zombie Army games