the bathroom bandit's greatest strength is his cannons but he can only use them if 1, he's alive and 2. he's ready for you, if you just snipe him or rush him without giving him time to react then it shouldn't be hard and the other bandits won't be able to put up much of a fight.
That's the last thing I remember, do you recall where in the novel that took place? I remember being weirded out by that theatre dungeon and kinda giving up on the novel around there, but I feel like going back now.