Here is the full list of all 20 N.O.V.A. 3 achievements worth 200 gamerscore.
Wield 7 weapons in a match.
Capture 3 flags without dying in a match.
Kill a sniper with a melee weapon.
Kill an enemy with your last bullet.
Kill 5 enemies in a match while under 25% energy.
Win 5 consecutive matches
Kill a flying enemy.
Hijack a vehicle after killing its driver.
Be the best player during a match.
Kill 10 enemies with the Mech.
Complete level 2 in under 20 minutes
Get 10 headshots with the sniper rifle in a Campaign level
Complete a campaign level without ever retrying
Kill one enemy with the sniper rifle without using the aim
Have your HP reduced to less than 10%, avoid dying and recover to full HP
Finish a level with an accuracy higher than 60%
Complete an entire level using only the pistol
Buy all the upgrades available in the campaign mode
Kill at least one enemy with each weapon of the game in the same level
Complete the campaign mode