Just BONESAW them into oblivion. It's not that hard!
I guess I could beat the normal bosses rather easily, but I'd have to learn their weak spots again, and especially that penguin kicked me down easily several times. I just don't feel like retrying a dozen time. Also on the first encounter I've beaten some bosses only by using the Bonesaw, so I haven't even learned how to defeat them normally in the first place. I could use the Bonesaw, but that would only help for one boss, or two if I'm quick. As you said the final boss is also hard, I suppose fighting him would be even more frustrating, so I guess I'll just be a coward now and give up.
Reminds me of Actraiser, which I nearly finished not so long ago. I had big trouble beating the bosses, but at least there are special spells you can use. Most are useless, but using one or two you could pretty easily defeat each boss. But alas, in the final level you have to fight each one of them AND still defeat the final boss after that, which was simply impossible for me. I could only use the spells few times so I would be able to beat one or two bosses with it, but not all 4 or 5 and then after that even the hardest one.
Difficulty curve and all, yeah, but I don't get why so many games have to be a lot harder at the end than they were before that, making it either impossible or extremely frustrating to beat the game if the difficulty was already rather high for you before that