The best tip I can give for this segment. Follow Dubbleups guide but one deviation.
On the third wave, do not order your men to cut the boulders until the RB button comes on screen. Right when the RB button comes on screen order 6 men to start on the left boulder, then hit the RB button. When that little mini cutscene is over then start the chant. This should give the trolls enough time to come up the hill and run right in the path of the boulder.
After those are taken care of then just guard that cross from any that stand in your way.
After about 10 tries I was able to finally get this one done. Of course it wasn't after a few beers either. hah. Man this game blows.