It is stated that particular NPC's health is huge for its level. It also uses two enchanted weapons.
Its level is radiant meaning that if you leave it, its level will increase higher. If you exceed its current level, then Kvenel will move up to a maximum of level 30.
He's immune to frost and poison. There is a conjurer associate with him, its recommended you kill him first to avoid him summoning his Frost Atronach.
If you're level 17/18, this quest shouldn't be too difficult for you unless the game has shafted you over and spawned the boss at max level.
Have you upgraded your Perks accordingly? Are you using upgraded weapons/armour? Are you applying appropriate buffs (spells/potions/scrolls) prior to combat?

