I did not become that thing, and I think you actually hit it on the head exactly:
"Could you, if possible, tell a bit of why the final boss felt like nonsense? We didn't manage to test him with other people and to us he was kinda easy to snipe, so maybe I went a bit overboard trying to avoid that."
It's trying to dodge everything he's throwing at you while trying to hit him and not the main (subset of the 'everything' I'm referring to) things he's throwing at you. It probably stacked with some of the controls stuff and ended up feeling a little trolly.
That said I still quite liked the game overall, which hopefully was obvious from my review.