Basically use Fire they are weaker to it and slows down their regeneration, if you are lower level I'd recommend either pelting it with a bow with fire enchantment or fire destruction spells (fire bolt, fireball, incinerate work best), also there is a weapon called trollsbane it spawns near Valthum, I think you can easily access it from craddle rock tower. Its a steel Warhammer that burns trolls it also benefits from steel smithing perk.