He is very strong. Like me. It takes a little bit to kill him from what I have heard.

did you use dragonrend on him to bring him to ground? after you do that you have to sword/bow and arrow him to death

Keep using dragonrend on him while the other dragon hurts him, while each dragon are killing each other, hide behind a rock or something with a bow or something and keep shooting him.

Thats what i did

And like i said before, the key is keeping him on the ground.

I had the same problem, I was a mage and very easy to kill. What I did was basically hide behind the wall with dragon writing on it where Paarthunax usually rests and shoot out a bolt of lightning whenever I had a chance. Eventaully I got to the point of saving after every successful attack I hit him with and survived his counter attack. I also realized that since I had the duel wield spells and impact perks, I was able to stun him for a moment everytime I hit him with a duel wielded spell. It was a hard fight but as long as you don't give up, you should be fine.

if your desperate and you know he just won't die lower the game to novice difficulty through settings and gameplay and difficulty if you can't kill him then just quit the game I'm sorry but youd have to be bad