Level 10 is pretty reasonable, Duran learns new healing skill, Also Treephop's Claw mastery makes him hit more in basic attacks, if you are still having trouble buy Treephop a Flaming Bronze Claw in Icywland, it'll dish a lot damage to Zulwhern. Also yeah Zulwhern's attacks becomes more deadlier in low hp, make sure to use your hp potions when Duran can't heal.