i've always used a tactic much like Taramoor's brother, only backwards. it's better to be a monk with the healing skill (forgot the name) and Guts secondary ability. run like a scared little bunny for the first few minutes of the battle, using yell constantly. then when you get 4 or 5 turns to every one of Weigraf's, start using accumulate. I always camped out in a corner and waited for him to get close to me before moving to the other corner. once you're tired of that, kill him in like 2 hits and watch as he changes to Lucavi. kill the other two daemons he summons to help him for more exp, then spank him like a 4 year old in K-mart.