Who in the world designed that one challenge where you have to beat 100 man Smash with Ken, but you can only use special moves? That was the most aggravating thing in Ultimate so far, especially since items and floor attacks count against you, so God forbid you accidentally hit a POW block that suddenly spawned in when you're in the middle of a Shoryuken. And even more irritating is that non Mii CPUs can pick up items, and heaven forbid they drop it on the battlefield, because one hit and it's back to the beginning.
Ugh, it took me hours to complete that. Especially since Shulk and Chrom spawned in in several of the games, so I had nothing to do against their range and (mainly Buster Shulk's) damage output.
Just ugh.