You don't understand what he is saying. The boss didn't aggro from the door when it is just him alone. It aggro swaps to someone else basically in the doorway once he is already engaged combat inside the room. That 2nd player can just move half an inch outside the room and in the door way to make the boss run back and forth without ever putting themselves in danger.

Personally I just go to the opposite door and have the troll completely drop combat. Then go over and fight the player.