For clarity, we are talking about EK?
Trolls will only permanently die if the killing blow has fire damage (fire damage from a weapon, spell, etc). They'll take damage from anything per their resistances, but if the killing blow doesn't include fire damage, they regenerate.
This can be problematic if you and your companion don't have fire damage.
Also note, some skills such as whirlwind, don't apply elemental damage from a weapon. So you, or your companion using whirlwind to land a killing blow would result in a troll regeneration.