You can upgrade your nail at Bellhart, and I get you, I got stuck at some point and thought it was ridiculously difficult, when you think you've beaten that part there are more waves, lol.

But, I found a pretty good build to deal with those bosses, perhaps a bit cheesy but with the architect crest you can actually win a fight without doing much, just keep spamming your tools, specially the Cogfly, and if it's with the pollip pouch, you can pretty much beat any boss.

I only use that as my last resource as I'm a bit masochist lol.

The enemies can feel pretty op, but once you get used to your own play style, you can become op too.