I’m dropping between 500b and 5T damage and still can’t beat Mephisto

So I rebuilt my warlock today from a dread claw build to a Hellfire build. Finally feeling powerful, I can spam the apocalypse ultimate too now. And that’s where I get my bulk damage from, best so far was 5T but I get 500b consistently with it.

Still can’t beat Mephisto, I’ve been one shotting some bosses completely. On tier 10, a two tap at worst. But for whatever reason, I just can’t stack numbers on him. I think it’s because my build is great for add clear but not single opponents, definitely shines brightest when I’m getting swarmed and just blank everything on screen at once.

Just hard to imagine why I can absolutely crush everything but him.

I haven’t looked up strategies and builds specifically for him yet, but the hellfire build definitely has me finally feeling powerful on my warlock. Was a disappointment till today.