There are a couple things you could do. First of all, if you are only 10 hours or less into the game then I would just say keep playing, do quests for mirian and increase things like your max health and such to make him easier to kill. Another tip is that he may only have 1 weakness, but unless his strenghts say he is invulnerable to ranged / stealth / combat finishers, then you can still use those to weaken him. And, use his weakness! Lure him to a fire and detonate it. That will also help kill any supporters nearby.

I've been playing the game for a little while and I'm pretty good at it now, can take down camps easily but this guy is just scared of explosions, they don't weaken him and he is a combat master and invulnerable to ranged attacks and stealth. HELP