1 shotting the boss with broken builds is part of the endgame experience. It’s a lord of hell but whatever, Nephalem are like that I guess. Lore wise just 1 shotting lords of hell. Just a disrespectful open hand slap and down he goes. Makes sense.
Or you could tone down the broken crap and buff difficulty in some way.
As of now “end game” is where the game ends. Not where you keep playing.