This is your first encounter with an enemy that can Diamondize you, so you'll want to defeat it before it has the chance to do so. Trillionage Sprout is weak to fire, so Paula should be doing nothing but her highest PSI Fire for this battle, assuming she isn't Mushroomized. (Using offensive PSI when Mushroomized is very risky, as there's a decent chance you'll attack your own party with it.) If she is Mushroomized, have her perform standard attacks and have Ness use his highet PSI Rockin (unless Ness is also Mushroomized). If everybody is Mushroomized, this battle essentially comes down to luck; hopefully you'll be able to do enough damage to Trillionage Sprout to defeat him before you do enough damage to your own party to lose the battle.