Shrooom! guards Rainy Circle, which can be found in the cave near Dr. Andonuts's lab in Winters.

One of the most annoying things about Shrooom! is that it can mushroomize you. And it usually tries to mushroomize an ally on its first turn. If the mushroomization attempt is successful, don't let the mushroomized character do anything.

Shrooom! can also poison your allies. Poison can be healed with Healing Beta or higher, or with certain types of items.

Other than those two warnings, this is a pretty straightforward battle. Shrooom! doesn't use any PSI, so a Shield from Ness can keep the damage down.

Otherwise, just use your best PSI, and shoot a Big Bottle Rocket to make quick work of Shrooom!