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I cannot kill Rohen. Is there some trick to it?
This question / problem has been solved by DarrkPhoeniximage
No trick, you just need a serious army that can both deal plenty of damage and take a beating. Avoid the temptation of trying to focus on Rohen from the beginning, as he takes quite a while to go down and during that time all his friends will be tearing your army apart. It's one of the tougher battles of the game, so all you can really do is make sure you're well prepared when you go into it (Rohen's appearance is triggered by killing the second of the two large dragon creatures in the area, so make sure you've got your army at full strength before doing so).
cnick501: I cannot kill Rohen. Is there some trick to it?
Well you can't kill him before aliyah (or something) comes barging in with reinforcements.
I used a lot of ranged fighters throwing ice rocks at his face.

I was playing a heavy warrior avatar so i could stand ground while the hurlers pounded the dino's. Then rohen can be killed the same way.
Make sure you use the H-button to avoid half of your army wandering around during the fight.