To beat the first galaxy boss (Jr bowser robot), you must crawl up Megaleg's ramp (in between the two feet). Then, you will eventually reach the head, where the grand star is under a glass dome. There should be two bullet bills firing somewhere, and if you let them see you, they follow you, and if you jump over the gate blocking the dome the bullet bill will follow you and smash the glass dome, freeing the grand star. Presto!