Here is the full list of all 12 Rekoil: Liberator achievements worth 400 gamerscore.
Awarded for killing 100 enemies with grenades.
Awarded for killing 1000 enemies while using any weapon from the Assault class.
Awarded for killing 1000 enemies while using any weapon from the Recon class.
Awarded for killing 1000 enemies while using any weapon from the Heavy Gunner class.
Awarded for killing 1000 enemies while using any weapon from the Sniper class.
Awarded for killing 1000 enemies while using any weapon from the Shotgunner class.
Awarded for killing 250 enemies while using any weapon from the Rocketeer class.
Awarded for killing enemies by Headshots 500 times.
Awarded for killing 10,000 enemies.
Awarded for capturing the briefcase 250 times.
Awarded for getting 250 captures in Domination.
Awarded for receiving a 15 kill streak in a single round.