The Pikmin resistant to the burns are red Pikmin, of course, and yellows can resist the zapping end. But only purples are burly enough to damage its metallic hide. When it slams down its head to try and fry Pikmin, make sure that it misses and then smash the head in with purple Pikmin. After the bulb falls out, the whole thing will shatter and explode. The bulb will then crack open, revealing the Dark Spore, and the huge room will fade out to a standard cave chamber with a geyser you can use to escape. It turns out the whole thing was just a hallucination produced by the Dark Spore.