I don't know what to do, I tried potions and all signs even putting oils on swords and this damn monster doesn't want to take almost any damage at all!

Just place down Yrden signs and lure her into them, occasionally they'll stun her for several seconds as opposed to 1 second. When this happens roll (upgraded helps) behind her and spam strong attacks on her back using your silver sword.

I see, the back not the front :P thanks

Most enemies will take more damage from the back. There is a damage penalty, and they're usually unguarded.