Better Than Harsh Language is a challenge in Aliens: Colonial Marines
Kill 5 enemies with melee in a single campaign mission
Chest Emblem: Cross My Heart
Chest, Leg Accessories: Survival Knife
Try equipping a grenade by hitting left on your dpad and then just duke it out Rocky Balboa style for the melee kills. That or lowering the difficulty setting. It's a tricky challenge
Also you can try the smartgun's melee attack. Not only is the range a bit better, but you don't accidentally use the pistol fatality.