Just hit 120 on paladin it'd hard to find video about paladin cr (do I use cr when everyone hitting 1 1 or something )
I may be wrong, I think total crash negates magic cancel as well, so if you see the magic equivalent icon, it would be good to cr if there is mage in the party. Also it is usually beneficial to pre emptively cast crash!
personally I like to CR as long as a boss can cast CR, not only when its active. its more efficient and you are less likely to forget / fall down (in HT for example). its mainly on zakum bodies, krex 1st eye, all the time on scarga and HT heads, also nibergen and I'm probably forgetting something else.
Technically APQ boss? And pianus but that's not real bossing and usually done solo. So I think you got them all
Mages can just switch to wand/staff whacking once magic cancel comes up, dealing the same dmg as their magic attack, nothing can stop mages from dealing dmg.