With a lot of cursing, I also managed to beat the third boss:
The bosses design is great, but they have too much HP, or the character doesn’t deal enough damage. Instead of being a glass cannon, like in many other Souls-like games, you are taking potshots at the bosses most of the time.
There are tons of sidemissions, which occur on the same levels as the main ones, but at different times of the day, and with different starting locations and blocked paths.
Must say that I don’t particularly enjoy that game, though. Apart from how fragile the hero is, you also need to manage all those weapons that are dropped randomly, which is simply not fun. A shame, I liked the character design a lot. But life is too short.