Finale Ribbon

As I understand it does a 4000% hit then debuffs a monster. What happens next though? Does it give a buff to the party making them do 15% less damage but raising their cap to 1mil(this has to be a typo right?), or does it make any damage the monster takes 15% less but he's able to take 1mil or whatever the cap is increased to?

Looks more like it debuffs the monster's attack to me, so you take less damage. It sets the cap on that monster to 1.2m at max level also, which applies to anyone, even if they're not in your party.