Yasuo vs Nasus

500+ stacks Nasus should win, unless Yasuo gets to insanely kite with the minion wave and does too much free damage. But I'm not sure. If Yasuo gets to have a full anti tank carry itemization with a bit of armor, it's not too easy for Nasus. On Nasus I'd get + either another armor item or if Yasuo takes lifesteal and armor (debatable because it has antisynergy with his E, but the healing reduction is extremely helpful)

And for Yasuo I'd get

Infinite stacks+full armor/health build wins.

In the most unlikely situation with the perfect amount of minions to work around Yasuo, otherwise Nasus.

Sorry, but Wither would Dumpster Yasuo hilariously. It increases the cooldown of his Q since it's tied to his Attack speed, his dash speed would be as slow as flowing honey, and his base damages wouldn't even dent Nasus's armor, not to mention Nasus's life steal from Qs. Yasuo has a bad match-up into Nasus to begin with, but late game, after all of Nasus's level ups and Armor and stacking, it goes from bad to worse

yasuo would beat nasus easily late game (assuming full builds)