Not sure if this has been fixed, but after release there was a bug where if you died during the fight you would be unable to kill him afterward because Svana wouldn't spawn and you wouldn't get the necessary synergy to finish him off. If you've died during the fight and that's the issue you're having, the only fix as far as I know is abandoning the quest and restarting (and it restarts from confronting Svargrim in the Blue Palace so it's a long one) and then making sure not to die.

If that's not the problem you're having and you're just having difficulty getting his health down, all the usual advice applies: eat food, make sure you have potions handy, slot a heal, run around and kite him, etc.