When weak and first going into the BF crypts I tend to put fires in hallways and let them kill themselves on it. Just keep the fire repaired. If they don't kill themselves then they will stand there and let you shoot them.

You just have to watch out for smoke.

Once you get some troll though it's easy enough. Just don't open doors until you are ready to clear behind it. Clear areas before opening doors to prevent backspawns.