Lately, I've noticed a very effective close quarters tactic that is very annoying.
Use Marathon, Lightweight, and Commando perks and equip akimbo Rangers, then just run around like a maniac and strike everyone down with a single Ranger shot or knife them.
I haven't done this myself, but have faced a couple people doing this, and it's annoying. They're so fast that you have little to no time to react when they appear around a corner in a closed-in area.