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.
Save the man in the hospital
Revive other players 50 times
Sever 10,000 limbs
Kill 50 enemies that are actively attacking other players
Kill 20 Elites in co-op
Kill 50 Super Elites
Get at least 20 for 1 with an explosive kill
Kill a total of 20 enemies while incapacitated
Get a 5-for-1 kill with any sniper rifle
Complete any Campaign chapter in 4-player co-op at any difficulty level
Successfully complete Army of Darkness
Successfully complete all Chapters on Sniper Elite difficulty
Kill 25 Summoners
Successfully complete The Keep
Headshot a Heavy Armour Zombie without removing his helmet
Successfully complete Forest of Corpses
Kill 20 enemies as they revive
Kill 100 Zombie Snipers
Kill 5,000 enemies of any kind
Successfully complete Freight Train of Fear
Successfully complete City of Ashes
Get at least 15 for 1 with an explosive kill
Kill a Survivor
Successfully complete Tower of Hellfire
Kill 500 Skeletons
Successfully complete Terminal
Successfully complete Crucible of Evil
Pick up a phone
Pick up a Panzerfaust
Successfully complete Gateway to Hell
Successfully complete Purgatory
Become possessed
Successfully complete Subway to Hell
Successfully complete Library of Evil
Successfully complete Labyrinth of Death
Reach a total shot distance of 50km
Successfully complete Cathedral of Resurrection
Kill an Occult General
Reach a Chain Count of 20
Kill 50 Suicide Zombies
Sever 1,000 limbs
Loot a Super Elite's corpse
Get an unbroken 10-headshot streak
Defeat 500 enemies with headshots
Pick up the Preacher
Get at least 10 for 1 with an explosive kill
Successfully complete Village of the Dead
Kill a Suicide Zombie by hitting his grenade
Kick 50 enemies down
Kill a Zombie Sniper in mid-air
Reach a Chain Count of 10
Kill 25 Fire Demons
Loot a Chainsaw Elite's corpse
Score 5,000 with a single sniper round
Reach a Chain Count of 30
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