Take the Necromancer skill Insidious Parasite or similar, and an interrupt skill (e.g. Distracting Shot). Cast Insidious Parasite on the bear so it hurts itself and heals you, and use your interrupt skill to prevent it from casting Troll Unguent. When using Distracting shot, remember to change from your bow to the Club of a Thousand Bears before finishing the bear off.
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