If you're on an organised team, Ambush is arguably the best map on the game. It's almost a perfect map. If you start on the A side of the map, head towards the central road where B is. One breaks left and takes position on the elevated position by the oil tank. One breaks right and goes into the sniper building, and the other gets themself on top of the buildings inbetween overlooking the middle of the road. From these three positions you can completely pin down all three of the elevated positions on the other side of the road. The other three team members get onto the road at B and just lie there covering off each of the three roads leading out of their side of the map. The only realistic way to stop you is by getting to elevated positions to pick you off, but your other three team mates are covering that off so they can't get into position.
We've managed to pull this tactic off a few times and it literally turns into a turkeyshoot!
I think the most I've got with the tube is three people, but last night on District I was a man possessed protecting C. Took out two coming around the corner from A. Took out another one who'd come through the courtyard from B, and then moments later (obviously after they'd respawned) I was on the verge of being pushed out of the area, so, I left them a parting gift, a frag directly on C. '40 points' popped up, got four of the buggers!