i literally can't get past the first fight

you have to watch his moves carefully, when he puts up his wand and does the awkward stance hes blocking and you cannot hit him, when he is making a portal on the ground he is doing a special attack or something and you need to block in advance as that is his most powerful move i believe, and when he is idle of course you can hit him.

Also make sure to rampage the cupboard as much as you can. You can find health potions there. 

absolutely terrible way to start the game

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