Why does everything instant kill in the late game?

I'm to some pretty high tier maps. I've got full resists, high armor, high evasion, 4.5k health, a good build allowing me to kill things consistently, damage converted to health, elusive on crit, and bosses are still pretty much 2 hitting me without warning. Is this just a thing? Or am I somehow still doing something wrong?

edit: I realize the title might be misleading, I do not think that these bosses are instant killing everyone. They're killing me. And apparently my armor and evasion aren't high I just thought they were because it seemed like a high number.