Ah, that guy. You are not supposed to kill it that early. Just skip it until you get powerful and come back at a later stage.

Or do what I did. I bought 100 arrows and shoot him through that cave entrance, he couldn't fit inside the narrow door. I thought it would drop something cool but all he dropped was his weapon that you can't use because you need a lot more strength lol

This game teaches you to skip these optional fights or you lose motivation to play.