Congrats!
I've completed all the challenges, 100% World of Light and I'm missing ONE spirit, being the Partner Eevee.
Have to say, no real reward or prompt for completing all challenges, but hey ho, it's the first time I've bothered to do it in a Smash game so I'll pat myself on the back
I managed 9.9 pretty easily with Mega Man and Lucina - the only point where I struggle is with Master and Crazy Hand, since I always fall for some cheese. If you're fighting a regular character as the final boss, it really isn't hard to do.
As for the partner spirits, I heard somewhere that they will be released to everyone at a later date?
I hope soon, because that 1 empty slot is fueling my anxiety