Here is the full list of all 20 Contract Killer achievements worth 200 gamerscore.
None killed when mission timer hits 0:00
Killed 25 Enemies without Misses
Kill 100 Bosses
Kill 100 enemies
Reach Level 20
Score 500 headshots
Die 50 times
Reach level 40
Unlock 10 informants
Buy 10 weapons
Unlock All Ammo Types
Reach Level 10
Kill 3 Enemies with One Shot
Fire 1000 Rounds
Capture 500 Enemies
Score a Headshot While Target is Running
Receive x3 Multiplier 25 Times
Use 100 Medkits
Kill 13 People in 15 Seconds
Receive 7 Daily Bonuses in a Row