At first the first two bosses I encountered (floating rock boss and the sewer serpent) were agonizingly difficult but then I found that there are cheap ways of dealing with them that make them incredibly easy... and I think it was done deliberately.

Unfortunately I have no idea what cheap method to beat the undead serpent boss is... Yeah, those are really irritating not so much because of difficult but because getting back up is so tedious, it's like you're being punished for not being perfect.

I agree with that. I defeated first stone boss at third attempt, cause I found easy way. I had sad times with giant green wurm, but when i figured it out, it was peace of cacke. I think difficulty level is just good balanced.