The Promethean Surprise
If you can find yourself a Shotgun or Scattergun somewhere on the battlefield, there's no better armor ability to have equipped than Promethean Vision. Once you've got your buckshot loaded, find a heavily trafficked corridor and wait around the corner. Crouch down so you don't appear on radar and hit the left bumper when you see someone approaching. You'll be able to pop up just as they turn the corner and surprise them with a face full of lead.
Should your enemies get wise to this strategy, be sure to relocate. Keep moving about the battlefield and popping up like a god-damned rabbid prairie dog. If you notice your enemies are taking wide turns around the corner, lay low. The shotgun's not going to help you at long range so either don't blow your cover and get the drop with a long-range weapon or just let them pass by and wait for your next victim.