Aribeth heals herself with a heal spell. Find a way to prevent her from casting. If you're melee, try knockdown. If you're a caster, try a silence spell. Or find a way to stun, daze or hold her. She's not all that tough. Worst case scenario, she can only cast heal 2-3 times, so just keep beating on her. Use a Greater Swordsmans belt if you can't beat her any other way.
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