Guys, i solved the Cruel Smash challenge. 2 i got with a few different fighters with enough experimenting, 4 remained out of reach, until...
Little Mac. He's fast enough to outrun them on the ground, his Over-B can sail right through them (though mistime it and they'll punish you fierce), KO if you live long enough (and rack up enough damage on them. The bastards are resilient enough that they will survive otherwise), but most important of all: counter. It does strike back with proportional force. Those annoying OHKO attacks they can throw at you? Right back at 'em. You don't even have to be any good with Little Mac. Lord knows i'm not.
Ike didn't really work with the counter, too slow. Lucario or Greninja might fold the same strategy better.
Monster Hunter: pissing me off since 2010.