Easy way to get Fast Kill to 100: Go to Don't Touch My Gems II and equip the Minigun. Fire until you feel like you're about to run out, then hit 'C' to switch over to Browning, and repeat the process until you're completed the challenge.
Get the one to the left of the silver one, and the one up left of that one. It should be reactive armor and hazard suit. If they aren't, you got the wrong ones...