When these were announced way back when, I initially believed they would be these mini-dungeons filled with trap mechanics/area-damage you need to avoid. This eliminated talent specs, since every spec would be able to move and avoid damage.
Instead, there were mini-dungeons where the bosses have area-damage you need to avoid, while the monsters leading to the bosses are over-tuned and are serious threats. Certain talent specs will excel, while others will struggle or fail.
The monsters leading to a boss don’t need to be a threat, they shouldn’t be the threat. The threat should be avoidable damage so all specs can survive without major healing. While some spells they cast can be interrupted, they just use it again before your interrupt cooldown is up. The auto-attack damage should be barely a problem since again, they’re just your basic enemies, they should not be a threat.
The best example I can think of are the Souls games. All the damage is avoidable, you can dodge, roll, block, or simply move out of the way. All specs can heal, be it with a flask or a healing spell. The bosses hit hard, but you have so many ways to mitigate it. While the enemies leading to a boss remain a threat, again, you have so many ways to mitigate the threat and you can easily dispatch multiple enemies without being hit. Their attacks don’t automatically hit you.
The key point is why do the monsters leading to the bosses hit so hard?
While I’ve learned to just let Brann aggro everything and pull one at a time (if it’s allowed, one of the delves today has them always come in packs), I did find a secret treasure and the monster was auto-attacking me for over 1 million damage and it was hitting really fast. It was impossible to kill so I just left it.
The bosses are so perfect, at least, the ones that don’t have an AoE that can’t be avoided (the scarab lord and his swarm, you can’t range it, you have to use defensives). The big boss himself (Zelvik or whatever) is great. It’s just, get rid of the little monsters that hit so incredibly hard. They don’t need to be that overtuned. Make them literal fodder.