Combined Arms is a challenge in Aliens: Colonial Marines
Use the Combat Pistol to kill 10 enemies that were on fire
Chest Emblems: Megaton, Bad Hombre
Combat Pistol Skin: Sand Blasted
Use Flamethrower attachment on Assault Rifle to set the Xeno on fire, then switch to your Combat Pistol to finish them off. The Xenos do not have to be on fire at the exact moment you killed them, its ok if they were on fire at one point and are no longer on fire.