In my opinion, immunity and one-shots can be okay, but only if it's to emphasize the necessity of actually thinking. The level 0 challenge is the perfect example. It is not a challenge of dealing damage or soaking damage, your gear is completely irrelevant, but rather just of moving while fighting. Making sure you have the situational awareness to destroy the ring without getting caught by the lazor. It is purely about skill (unless you're playing paladin, then you can just run around and let your vessel do all the work so long as you dont run into the beam). The countess falls in with that, the one-shot having a warning to it that you have to watch out for. The butcher's a bit less so. There's a condition you have to play with to get him to stop being invincible, but having the condition be "get hit but don't die" just doesn't feel satisfying. Meanwhile Act III's Ni Zas, which just randomly make everything around them invincible, is just aggravating. If you're a ranged class it means you just have to wait for the shield to wear off, and meanwhile if you're a melee class it means you have to focus on not dying for a bit. See also: The walls made by svartalfs, also in act 3, it does exactly one thing: makes you wait 5 seconds for it to go away, because generally they're not even on-screen when they cast it.