Sam's boss fight is required to progress through the game. He's relatively simple, and just hitting him and parrying him should do it for most of the fight. When he doesn't have his sword, he will throw your ass when you try to hit him with falling thunder. If you're not careful he will clap you. But if you are careful, you can stab him and kill him. You can't steal his sword just yet, but soon enough you'll be able to. He also has DLC or something but who cares