Here is the full list of all 68 Zombie Army Trilogy achievements. It takes around 30-35 hours to unlock all of the achievements on Windows.
Kill 50 Super Elites
Kill 5,000 enemies of any kind
Score 5,000 with a single sniper round
Kill 25 Summoners
Kill 100 Zombie Snipers
Revive other players 50 times
Successfully complete Army of Darkness
Successfully complete The Keep
Kill 20 Elites in co-op
Headshot a Heavy Armour Zombie without removing his helmet
Successfully complete Freight Train of Fear
Successfully complete City of Ashes
Pick up a Panzerfaust
Save the man in the hospital
Become possessed
Successfully complete Forest of Corpses
Loot a Chainsaw Elite's corpse
Reach a total shot distance of 50km
Kill a Survivor
Successfully complete Tower of Hellfire
Kill 500 Skeletons
Successfully complete Terminal
Successfully complete Crucible of Evil
Pick up a phone
Successfully complete Gateway to Hell
Kill 50 enemies that are actively attacking other players
Successfully complete Purgatory
Successfully complete Subway to Hell
Kill a total of 20 enemies while incapacitated
Successfully complete Library of Evil
Defeat 500 enemies with headshots
Kill 50 Suicide Zombies
Kill a Suicide Zombie by hitting his grenade
Sever 1,000 limbs
Successfully complete Labyrinth of Death
Complete any Campaign chapter in 4-player co-op at any difficulty level
Kill 20 enemies as they revive
Get at least 20 for 1 with an explosive kill
Get at least 15 for 1 with an explosive kill
Get at least 10 for 1 with an explosive kill
Successfully complete Cathedral of Resurrection
Kill an Occult General
Kill a Zombie Sniper in mid-air
Successfully complete Village of the Dead
Loot a Super Elite's corpse
Pick up the Preacher
Kick 50 enemies down
Reach a Chain Count of 30
Reach a Chain Count of 20
Get an unbroken 10-headshot streak
Reach a Chain Count of 10
Successfully complete all Chapters on Sniper Elite difficulty
Kill 25 Fire Demons
Get a 5-for-1 kill with any sniper rifle
Sever 10,000 limbs
Put the tortured souls in the Forest out of their misery
Shoot the 3 Zombie Ravens outside the Folterschloss
Reach at least Wave 10 in Horde Mode – Waves of Despair
Reach at least Wave 10 in Horde Mode – Cathedral
Reach at least Wave 10 in Horde Mode – Flood of Tears
Reach at least Wave 10 with the highest score in any Horde Mode map
Reach at least Wave 10 with more gold medals than your companions
Complete every Survivor sidequest
Complete levels as every playable character
Collect every Gold Bar
Find and shoot every Bottle of Blood
Reach Wave 10 in Horde Mode – Dead End
Reach Wave 10 in Horde Mode – Dead Man's Bluff