After you`ve gone through the swan mainenence room, there is a hole in the floor, which is a point of no return. If a Tank spawns there you are virtually dead. I`ve just survived TWICE on Expert level with Bots. The idea is to back up as far as possible, firing as you go, then qucikly run around him and melee from behind. I managed to kill him on both occasions with only one Bot capped.

Sorted!