I can't seem to get Gattuso knocked down for the secret mission, any advice?
Fight on top of the flowers. Hit them and they'll release a gas that will stun Gattuso (and anyone near them).
If you don't want to be stunned too, you can hit them with a ranged attack like Azure Edge, but Gattuso has to be near them.
If you're having difficulty with Gattuso itself, just have Estelle use only First Aid, Rita only use Fireball, and spam Destruction Field (gotten at level 16) with Yuri.