Here is the full list of all 36 Mercenary Battle Company: The Reapers achievements.
Exterminate 1000 Creepers
Exterminate 1000 Apex Creepers
Exterminate 100,000 Creepers or Apex Creepers
Exterminate 1000 Eruptors
Exterminate 1000 Apex Eruptors
Exterminate 100,000 Eruptors or Apex Eruptors
Exterminate 1000 Drones
Exterminate 1000 Apex Drones
Exterminate 100,000 Drones or Apex Drones
Exterminate 1000 Spewers
Exterminate 1000 Apex Spewers
Exterminate 100,000 Spewers or Apex Spewers
Exterminate 1000 Slimes
Exterminate 1000 Apex Slimes
Exterminate 100,000 Slimes or Apex Slimes
Exterminate Apex Venom Wing
Exterminate Elder Apex Venom Wing
Exterminate 100 Apex Venom Wings
Exterminate Apex Molten Slime
Exterminate Elder Apex Molten Slime
Exterminate 100 Apex Molten Slimes
Exterminate Apex Mammoth
Exterminate Elder Apex Mammoth
Exterminate 100 Apex Mammoths
Survive any level with Difficulty 1
Survive any level with Difficulty 5
Survive any level with Difficulty 10
Survive all levels with Difficulty 10
Unlock 1 Mech
Unlock 1 Weapon or Mech Attachment
Unlock All Mechs
Unlock All Weapons
Unlock All Mech Attachments
Unlock Pilot: Little Monster
Unlock Pilot: The Tactician
Unlock Pilot: Captain