Ok, here's the best way to beat rock or earth trolls. Get in close and unload on them with a melee weapon. I know it sounds crazy but trolls are SLOW and you shouldn't be by the time you get to the arena. Here's the trick, using target lock, run in and hit the troll with a melee weapon, if you have slow time and/or muti strike cast it. A troll can't hit you with the thrown rocks when your right under its feet. When you see it start a jumping attack, dodge straight back twice, charge right back in as soon as the rock circle around the troll starts to fade and hit it again. Without the help of slow time, I usually get two standard hits and one flurish in between his jumping attacks. If your quick enough, and a little lucky, you can do it without taking a singe hit.
As far as the trolls in the arena go, don't stand in between to two trolls or the one your not attacking will still hit you with thrown rocks. Let Whisper go after one and you take the other. Get on one or the other side of it and unload on him. The other troll may still lob a few rocks at you but Whisper mainly keeps him busy. DO NOT get between the troll and the rails, you won't be able to get away from the jump attack. Once you've dealt with one troll, go get the other.
This method DOES NOT work on ice trolls.
