When you meet Kahn for a second time in the Infernal Abyss, he will be plastered to a metal wall in a small room. Next to him is a small hallway that leads to four spawning pits for each Minion and one treasure chest. The treasure chest will respawn every eleven seconds after you hit or open it. Note: The Infernal Abyss is the fifth and last one.
This trick requires at least 10 minions. When you reach the troll, you will get a cutscene of him getting ready to fight you. After that, have all your minions rush to open the gate by the crane. If the troll is blocking the path, just lead him away, and go through the gate. The troll will probably kill a few of your minions as they open the gate; however, it takes approximately 20 to 40 minions to kill him on the Legendary difficulty. Once you open the gate, go to the crane to auto-save. After you auto-save, the game will assume you defeated the troll since you went further than you were supposed to. Simply leave the area and come back, and the troll will not be there anymore since the game thinks he has been defeated.