I went for lightning, all trolls seem to have a low lightning resistance.
Normal troll:
fire: 28
ligthning: -62
forest troll:
fire, poison:58
lightning:-32
the king is a bit higher. Prob is, he can cast Limbs of Lead. Try purple potions, they boost up resistances temporarily, place a teleporter outside the cave and vanish if you're in a tight spot. Reenter via stairs and target them again.
Shadow potions make invisible, then target the next troll by hitting NEXT to him, not on him. Maybe scorpions?
Kiya