Jotun is impossibl to solo cuz you have jotun. Plus2 other bosses and a Grendel sapscar
Jotun is not impossible to solo. You have to defeat the brothers in the other dungeons first to make it 'easier.' It's still a challenge, but I've beaten him solo on several wizards.
You're not doing it right. When you first entered, the welcome speech should have given you a strong hint that it's a bad idea to fight all three bosses at once.
Search around and you'll find a way to fight them one at a time. Soloing those fights is still hard, but possible. I always bring a friend.
You might want to try the other dungeons next to him first. If you kill of his brothers one at a time, then they will not appear when you try to kill off Jotun. Good luck!
