That fool joined me in the battle, but died early. Is there any way i can save his ass?

SAving Berengar is possible, althought, like KiNgBrAdLeY7 said - doesn't give anything. Players usually just treat it like a challange or achievement to save him. :)
Mostly... I think you need a resistance for knockback and some c.c. of your own... Plus big hopes that the boss focus on you, and not on Berengar.

Yes, i forgot to mention that, hey nice add!

Also, talents like the dexterity tree's one for resisting blinding, the werewolf mutagen plus trying to fight Azhar during night, as well as strength and dexterity trees' talents for dodge (there is one in each that gives 10% chance to dodge and they stuck), help to that extent, too! Just run up to him and tank, trying also to deal enough damage before he (berengar) dies.

Yes, willow is a must if you want to save Berengar.

It's hard work to distract Azar from Berengar. My best results I got using Aard - till the cow comes home:

https://www.youtube.com/watch?v=KBb-__E3Kyc

(DL: hard)

I did it! Nothing special, though, i expected some different quest entry or something like. At least i saved a life, lol! Appreciated, bros!

This is the most annoying challenge I have ever faced in the game. Azar often blindens me while he is lying on the ground but I never can use my magic when I am down/blinded... Usually he kills Berengar when he is on his last 10% of the health and if I manage to save Berengar his body drops in the canal... Then I have to reload and suffer through the veryl ong series of cutscenes with Berengar entering the battle quie pointlessly. He is no help and saving him is so much work. Azar does his best to ignore me after I am blinded and cannot stop him to save Berengar. And Berengar is so eager to rush between me and Javed preventing me in attacking him and getting himself killed. I have suspicion he wants to die. And half the time I start the battle blinded. I hate cutscene to battle thingies I like to position myself...
And when I finally manage to save Berengar and position Javed so his reamains will not fall into inaccessible area I get nothing. All the hard work and the only difference is missing journal entry about Berengar's death:/

And if you also use Devil's Puffball, the fight will be over in half a minute:

https://youtu.be/hJmpYhFmBhQ

(DL: hard) That's what makes it so appealing, especially if you're not playing the game for the first time: The challenge.