Anyone can be killed, even boss ; )
Perhaps you could use previous save.. or proper forum

Who is the first boss?
It should be the griffon, but I don't remember he casts a shield on him (even because it's an animal).
If otherwise you are referring to Vesemir in the tutorial, you can't kill him :)

You could try the game specific sub forum?

Kill all the hounds, then the shield goes away and you can attack him.
Repeat if necessary.

I am sorry i mean the first solider of the wild hunt :) i cant kill him even if i kill that ice dogs :D Its not working but really thanks for your reaction :)

Is this the fight with the sorceress helping you? If so, what you need to do is make sure he doesn't heal much. The longer you take to kill the dogs, the more he heals. It is, truly, a hard fight. I would just say use whatever buffs you have available to increase your damage, and hit him as hard as you can. You can't afford to let the fight drag on too long. Don't try to hit him when his sheild is up - kill those dogs as fast as you possibly can.