Just have bosses take a different fraction of dmg from cb. Deadly strike only works for phys, so elemental/poison damage chars don't benefit. Cb provides a common boost to both. Depending on the implementation for bosses (which probably inherit from a base monster class) it wouldnt be too hard to override the take damage method, and modify how cb effects bosses. Which is surely how they made it useless vs bosses at present. Problem is, it isn't intuitive to new players.
The cb animation plays but your dmg is ass vs bosses. Feelsbadman. At the very least, they should stop that from occurring.
I guess another consequence is cb feels useless, no one will bother getting it, so its just a wasted stat as 'straight dmg' will perform in all circumstances.