First you need to inflict it with as many bad ailments as possible, it's susceptible to tons of them. Then boost your party and make a strong assault against it so to harm it more than it heals up.
poison it, make it bleed, inflict it with as many status ailments as possible, it will lose thousands of HP just with that. then once it's alone and close to died, boost your characters and use warrior's acolyte (cybel) to pound the little creep into submission