Dutch hasn't the average human being, I'm sure his muscle mass shielded his insides a little, as far as the punches to the head, Arnie got a hard head but he got some brain injuries I guess, in the P2 novel it says he run off from the hospital and disappeared, in those comics with his brother he is also missing, maybe he went a little crazy with those concussions. It was like a grown man fighting a teenager girl, there is still a chance of survival.
But remember that this was the first predator apperance we had no idea how strong they were, it was only later on, mostly in the EU, that they started to get super strenght, like ripping off people's limbs, breaking people in half, crushing heads with their bare hands, in the first movie they weren't suppose to be that strong.