Private (Act 1) - 15 points: Complete Act 1 with the rank of Private
Private (Act 2) - 20 points: Complete Acts 1 and 2 with the rank of Private
Private (Defeated the Strogg) - 25 points: Complete the game with the rank of Private
Corporal (Act 1) - 25 points: Complete Act 1 with the rank of Corporal
Corporal (Act 2) - 25 points: Complete Acts 1 and 2 with the rank of Corporal
Corporal (Defeated The Strogg) - 30 points: Complete the game with the rank of Corporal
Lieutenant (Act 1) - 25 points: Complete Act 1 with the rank of Lieutenant
Lieutenant (Act 2) - 25 points: Complete Acts 1 and 2 with the rank of Lieutenant
Lieutenant (Defeated The Strogg) - 30 points: Complete Acts 1 and 2 with the rank of Lieutenant
General (Act 1) - 25 points: Complete Act 1 with the rank of General
General (Act 2) - 25 points: Complete Acts 1 and 2 with the rank of General
General (Defeated The Strogg) - 30 points: Complete the game with the rank of General
Untouchable - 10 points: Complete a level without taking damage
Blaster Master - 15 points: Complete a level using only the blaster
Hyperactive - 15 points: Complete a level using only the hyperblaster
Blazing Buckshot - 15 points: Complete a level using only the shotgun
The Machine - 15 points: Complete a level using only the machine gun
Tooth and Nail - 15 points: Complete a level using only the nailgun
Pineapple King - 15 points: Complete a level using only the grenade launcher
Ballistics Expert - 15 points: Complete a level using only the rocket launcher
Enlightening - 15 points: Complete a level using only the lightning gun
The Dark Lord - 15 points: Complete a level using only the DMG
The Sniper - 15 points: Complete a level using only the railgun
Full Arsenal - 20 points: Complete a level where a shot with every weapon hits
Solar Star of Courage - 20 points: Fight alongside Viper Squad (Hangar2)
Sky Marshall's Medal of Valor - 20 points: Rescuing Pvt Lanier of Eagle Squad (Waste)
Galactic Order of Heroism - 20 points: Fight alongside Raven Squad

Note: If you use a cheat, you will not get any points [thanks HokyPlayr10]