The following is somewhat of a spoiler, to avoid it being shown in the question excerpt, I'm filling the space with useless boilerplate.
I'm near the end of the first chapter and I'm now facing Letho, the kingslayer and witcher. And I'm getting my ass kicked thoroughly and completely. I haven't even been able to hurt Letho in the short time I'm staying alive.
He shields himself with Quen and just blocks every single attack I make. Then he blasts me with Aard, throws a poison grenade at me, or incinerates me with Igni. I really haven't been able to make any dent into his health bar, he just blocks every attack. And I can't keep up Quen as fast as he damages me.
Any tips on how to kill that bastard? I'm not kidding when I say that I see no way of surviving that fight at the moment.