Here is the full list of all 22 A Mass of Dead achievements.
Cleared All Normal Difficulties of Challenge Stages
Cleared All Hard Difficulties of Challenge Stages
Cleared All Hell Difficulties of Challenge Stages
First saw the ending
Get the first unique weapon
Get The First Unique Equipment
Full Upgraded All Equipments
Full Upgraded All Flamethrowers
Full Upgraded All SMGs
Full Upgraded All Shotguns
Got All Equipments
Got All Flamethrowers
Got All SMGs
Got All Shotguns
Full Upgraded All Weapons
Full Upgrade All Items
Got All Weapons
Got All Item
Cleared All Normal Difficulties of Challenge Stages in co-op mode
Cleared All Hard Difficulties of Challenge Stages in co-op mode
Cleared All Hell Difficulties of Challenge Stages in co-op mode
Cleared one stage in co-op mode