On Legendary, the first troll is very hard and consumes alot of minions. There's an easy way to completely skip him (Make sure you have 10 minions):
Head straight to the troll, and you'll get the cutscene of him getting ready to face you. After that, send all of your minions to open the gate where the crane is, and rush there (the troll will probably kill a few of your minions as they open the gate, but that's better than wasting 20 - 40 minions trying to kill him and also, if the troll is blocking the path, simple lead him away and head through the gate).
Once done, go to the crane and an autosave will occur. This is where the trick comes in: That autosave assumes you defeated the troll as you went further than you were suppose to, therefore, simply leave the area and come back, you'll find the troll has vanished as the game thinks he has been defeated.