Rummage the cupboard two day in row (for tow poison) before duel. First attacks, then block if he block, cast pentagram or summon demonic hand and attack if he stay. Best time for heal it when you in block after pentagram before attacks of demon hand. In final try attacks when he cast pentagram, it deals more damage and hand don't attacks you if you win in that turn.
Snape action is random so once i am win without use health poison