I wonder, they canot get frozen for sure but can you at least like immobilize, chill or slow them in order to make the % dmg against crowd controlled enemies work when fighting a boss or it is just useless bonus against boss cuz you just simply can't crowd control them?

All cc damage bonus is only active when boss is staggered.

And what is that mean exactly if you don't mind breaking this down a little... 😅

When your fighting a boss he can become staggered (blue bar appears next to his health and he is stunned), this lasts 10 seconds.

How fast/often he becomes staggered is determined by your build and items/skills and also the Crowd controlled duration stat on gear makes the stagger bar fill faster even tho it doesn't display this ingame...
So the more crowd controlled effects you do = the faster the boss becomes staggered.

When he's in that staggered state, your crowd controlled bonuses get applied.

I understand so much more now. Thanks bro 👍