Here is the full list of all 12 Contract Killer achievements.
Defeat Landlord
Defeat Loan Shark
Defeat Fax Machine
Defeat 100 Contracts
Play 10 Battle Matches
Revive 20 Players
Get to Wave 10 in Horde Mode
Play with a full party
Play an alternate skin
Defeat 300 Contracts
Blow up 20 barrels
Parry 25 times