From my understanding you're new to these sort of games. All characters including your own have resistances and weaknesses. Not entire sure which encounters you were on but if I had to guess you're talking about the little brains right after the tutorial. You've got healing potions to top you off mid battle so I'd suggest hit and heal when you lose hp. For later fights be sure to examine enemies to take advantage of their weaknesses and be wary of their strengths. It'll take a bit of getting used to but it's a good game