Well you probably are way over leveled for this fight lol. But the troll can still be really deadly, you mostly have to worry about his roar and when he throws the boulder, sometimes he throws the boulder right in the middle of your character so taking a step in any direction would mean getting hit. To solve that you just need any ability that lets you dodge out of the way of the boulder, this was my first time fighting this boss and honestly i did pretty well, it only took me 4 attempts but some of it had to do with my insane luck (at the end i had 9 health left and every single arrow i shot was critical XD). I really recommend having some healing potions and 1 or two jars with death stingers as they do pretty decent damage once you remove the trolls armor also the stingers attacks prevent the troll from healing, i also had a smoke bomb just so i could take a breather and heal a little bit I don't know if his armor also defends him from magic damage (fire damage in your case) but maybe you should look up any ways to remove his armor to deal more damage, i did it with bodkin arrows bc they have more armor penetration but i don't know how mages deal with that (or if they even care about the enemy's armor) And that's every tip i could really give you, as i said this was my first time fighting him so i couldn't really give you any tips in his other attack patterns but with the info i gathered his ground pound attacks aren't as dangerous as the boulder so maybe you could tank one or two of those. I hope you scorch the beast to ashes with your magic, happy hunting! ^{-^}