You will want to defeat the Tough Mobile Sprouts early on so they won't draw your efforts away from beating up Trillionage Sprout. PSI Rockin is a good way to do that. Paula can use Fire (it's more effective than Freeze), and as usual Jeff can help out with Bottle Rockets, and Bombs are also an option for anybody.

Just be careful because your enemies here can mushroomize you, so you don't want a mushroomized ally to use PSI or Bottle Rockets or what have you, because in their confusion they might target an ally instead of an enemy.

Otherwise, just attack away, and soon the path to Milky Well will open. After going there, you'll head back to Threed and make your way to Dusty Dunes Desert.