Here is the full list of all 20 Contract Killer achievements worth 200 gamerscore.
Killed 13 People in 15 Secons
Reached level 20
Captured 500 Enemies
Died 50 times
Bought 10 weapons
Killed 3 Enemies with One Shot
Killed 25 Enemies without Misses
Unlocked 10 informants
Received x3 Multiplaier 25 Times
Used 100 Medkits
None killed when mission timer hits 0:00
Fired 1000 Rounds
Killed 100 Bosses
Scored 500 headshots
Reached Level 10
Scored a Headshot While Target is Running
Killed 100 enemies
Reach level 40
Unlock All Ammo Types
Receive 7 Daily Bonuses in a Row