The most this technique requires is perhaps respecing. you may use any of your teammates as long as they are a decent level, and you can be any class. just simply get break and use it after he casts dark omen on himself. do not do anything except that. Omen casts dark omen on himself every 3 rounds which coincidentally is the cooldown of break. keep yourself and your teammates alive and let them do damage. this takes awhile, but is effective nevertheless.
You could cast Shatter Bolt on him, since that Dark Omen raises his damage quite a bit. Shatter bolt will stop him that turn and still deal some damage.
Actually, I beat him as a magic user so I didn't have break...Didn't actually think of that. Either or, I guess. Break might be more effective since I don't know the cooldown of either of them or how many times they can be equipped to the action bar. I also don't remember how much damage Shatter bolt dealt.
Another thing that helps is tog et verdux, if you're using veradux, paladin so his poison is way up. If he does use poison on Omen while he's has dark omen on, he'll do 50k damage.