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