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