maybe there is some secret to this battle? I easily kill his minions, but then the poison comes everywhere, and my PC lags heavily on top of that... Tried to kill the boss first but it seems even harder... Tomorrow I'll try it again on minimum gfx settings, if I fail the game will be over

UPD
and I'll take the chance to ensure I'm on right path:
The above fight is on level 15, seems there is nothing else besides it left on this level.
Prior to level 15, I visited level 13, where you find a lot of gems. Opened a lot of places, but could not solve the demon heads puzzle - I assume gems have to be inserted but I have only 7 or so. Was unable to find anything else I could do, thus I went to level 15 in hope to find more gems there or smth else.
Do I get it right - w/o level 15 you can't complete level 13?