For this achievement you must complete the challenge "12 Days of Pandora"
12 Assault Rifle kills
11 Pistol kills
10 Shotgun kills.
9 SMG kills
8 Sniper Rifle kills
7 Melee kills.
6 Critical Hit kills.
5 Explosive kills.
4 Shock kills.
3 Incendiary kills,
2 Corrosive kills
1 Grenade kill.
Progress can be checked in the challenges menu.