Tldr: Suggesting the addition of new delve zone abilities or delve-only useable items that interrupt boss casts for classes with no/long-cd interrupts. Share CD with built-in class interrupts.
As we had seen with Torghast at higher levels, certain delve mechanics strongly favor classes with a surplus of specific types of disrupt abilities. Particularly for bosses requiring frequent interrupts in order to survive. Some casters have long CDs on their interrupts (shadow priest, balance druid), or have to sacrifice core survivability for interrupts (warlock). Delves suffer more from this issue than Torghast did because the end bosses are immune to CC, which affects some specs far more than others. They cannot be snared and so kiting is not particularly effective outside of short bursts of mobility.
My ideal solution would not be to make the content any easier by reducing the frequency or lethality of casts, but to even the playing field between classes by providing tools to cover gaps. This mitigates the need to homogenize class talent trees further. An example tool would be an item, obtainable from delves and only usable inside delves, that interrupts your current target. It might have a 20 second CD and share CD with class interrupts. Alternatively, make bosses immune to standard interrupts but vulnerable to this item.