Magic Beast's defense is fairly low. You can hit for 400-700 damage with physical attacks or more than that with Beast Killer rings and Powera/Powerus. -a spells will do 600-700 damage, while Forcea does ~900 and -us spells do around 1,200-1,400. Just hit this guy with your strongest stuff. Neither Sonic Quake nor Ground Strike are dangerous. Flare Core will do 2,000-3,000 damage to everyone if he hits it. The bright side, is it will take him three turns to ready it, so you'll know when it's coming. Have someone cast All-Barricadus at the start of the fight, and have everyone defend before it hits. Then afterward, immediately heal. If you can live through that, there's nothing else the Magic Beast can throw at you that should be able to stop you.