Mathias told me about Gorefist, the first boss that he couldn't beat. I reached his lair in the dungeon today, and I see the problem. He killed me once, and then I went back and upgraded my armor and weapons. The Weapon Mods are really powerful, both Mender's Aura (healing) and Hunter's Mark (mark enemies, more critical hits). Going back in, he killed me 5 more times. I decided to go back into the dungeon and grind a bit more on the easy Hollows.