Once you enter stage 5-5, float above the doorway and you'll see what looks like a window with three bars on it. Press up to enter it as you would a normal doorway and you'll go through a much harder version of the stage. The four sections are 1) Water Galborus, 2) King Doo followed by Dubior, 3) Kibble Blade & Gigant Edge, 4) Bonkers & Moundo. In addition to being against harder enemies, the fights take place in smaller than normal areas. You can get the four Energy Spheres and the path is available on EX mode.