Where is your last save? Even if you can not leave to get more potions, a different strategy (as Dragonbane suggested above) may be much more effective than just rushing in.
The demon is easier to take down than Laiken, and Sassan can heal or summon rather than fight. Any spare skill points put into Life Leech may help (don't recall the level requirement), or a ghost or demon summon. With a Rush Attack you have a chance of stunning your opponent briefly, so even if you have to reload a few times you'll eventually get and extra hit or two in.
