Here is the full list of all 43 Foreign Legion: Multi Massacre achievements.
Host a 3 player game
Win a domination game.
Wear the BunnyEars and jump 500 times.
Honk your clowns nose 500 times.
Pop 250 pumpkins
Break 250 wooden boards
Kill 1000 zombies
Break 250 tombstones
Start the Generator 50 times
Break 100 tombstones
Start the Generator 25 times
Blow up 25 dolphins
Shoot 7 peacedoves
Pop 100 pumpkins
Finish a game in EXTREME
Break 100 wooden boards
Kill 250 zombies
Shoot off 250 zombie limbs
Break 25 tombstones
Break 25 wooden boards
Make 50 enemy headshots
Pop 25 pumpkins
Shoot off 100 zombie limbs
Kill the ghost on time
Kill 25 zombies
Shoot off 25 zombie limbs
Make 250 enemy headshots
Make 1000 enemy headshots
Host a 4 player game
Kill 25 players
Kill 100 players
Kill 250 players
Make 25 fueltanks explode
Make 100 fueltanks explode
Make 250 fueltanks explode
Shoot 30 peacedoves
Shoot 69 peacedoves
Blow up 100
Blow up 250 dolphins
Snipe 25 prairydogs
Snipe 100 prairydogs
Snipe 250 prairydogs
Start the Generator 100 times