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).