Do you have SW:BF:II installed on any other computer on your home network? I have a similar problem, with the same result. I have battlefront 2 installed on 2 computer's in my house... I have 2 discs for it, a DVD and CD version. Even though both keys are legit, it doesn't let both computers join the same game, unless one of them hosts. Try contacting LA?
If TK is off (I don't remember if it is in campaign) Then just lay down a det-pack with the engineer right before the master is on top of you, and then det it! Or, get close to them and blow them away with that uber shotgun! Do a side roll so you come up close to them, then boom!