Oracle's Drain spell works really well against him, it does around 300 damage.
The Lich summon should do about the same damage to Belias as Drain does.

Belias has around 900 HP if I remember correctly, so... with some luck and those spells, you should be victorious.

Oh yeah and I recommend Auto-Potion with only X-Potion in inventory on every character, it really helps.