Okay, okay, we got this!

Crispy, use a Smokescreen to make it harder for the plant to hit you. Next, just relax in the smoke for a while--no sweat. Finally, you and every one of your clones rush out of the smoke and engulf the entire area where Shroomish is standing with a powerful Flamethrower (just hits the ground where Shroomish is standing.. it's grass, isn't it? ^_^)