1. Keep bosses immune to instant death spells - we're fine with that. Phantasmal Killer, Finger of Death, Destruction, Slay Living. Flesh to Stone, etc.
2. Remove Boss immunities to all other effects. Replace them with innate resistance to those same effects.
Examples:
- You can trip bosses, but their save is high, and they stand up almost immediately.
- You can Stunning Blow bosses, but their save is high, and they are stunned only for a few seconds.
- You can debuff bosses, but their save is high, and they last only a few seconds.
etc.
This opens up a lot of new strategies for beating encounters without allowing players to trivialize the main boss fight. Spells/effects like Slow, Waves of Exhaustion, Blindness, etc. can be used to help reduce Boss DPS for a few seconds per use and allows caster types to be more diverse in their spell selection/tactics without having to "resort" to buff bots/haste bots/Max-Empowered-Disintegrate bots on the main bosses.