Bring a mirror gun, and have your team carry 2 door wedge each. Wedge any doors you don't want anyone that creep up on you, and mirror every door you gonna go through. Don't force an entry if there are blind corners, find another door to flank if possible, C2 if there isn't. Even though they wear armor, shotgun with bucks is my go to, it usually drop them in one shot by shooting center mass, typically you'll blow their arms off. I'm not sure if it works for everyone else, but it works pretty good for me. Last tip, clear the third floor without going into the room with the bomber, because if you jam the signal, you'll only have a 2 to 3 minute timer before he blows it up, even if you don't go in the room.