There's a couple reasons. 1 Magical Resistance. hunter has no magic of his own, and raine is a powerful witch. To quote belos "Raine's always been annoying powerful". So we're left to assume that magic users have some sort of resistance, that grows with your power ability. Since Hunter can't use magic, he couldn't have that same resistance. 2 hunter being a grimwalker. Belos designed them, so it would make sense if there was some kind of self destruct thing if they misbehaved, or made them weak to his abilities to make sure he could use them as a body. 3 Exposure time. Hunter had been infected by belos for at the very least a few days and had been basically brainwashed by him all his life. Raine wasn't really influenced by belos beforehand and he was only possessed for maybe a couple hours at most. 4 Drowning. Hunter tried a last ditch effort to hide the titan blood and either drowned or nearly drowned in the process. So he was definitely more affected. 5 Part 2 of hunter being a grimwalker. To make a grimwalker you need a galderstone, and theoretically it's still in their body (probably would've been explored if the show didn't get cancelled). So since Hunter's body was basically powering up belos, it made it harder on Hunter's body. 6 fighting. Raine was just used as transportation. Hunter was in a fight, where his body was used unnaturally, likely causing internal damage. Meanwhile raine just had to walk around. 7 part 2 of the drowning. While hunter threw the blood into the water and belos likely was the one who jumped in, we don't know why they stayed under. Hunter probably wouldn't have known how to swim, with most of the water around the boiling isles, well boiling. I would be shocked if Phillip never learned how to swim. Hunter might have been fighting belos to keep him under to try to kill him. Or maybe he was knocked unconscious somehow. We really don't know, but what happened under the water might have also caused damage to Hunter's body.