Topmaniac is a sub boss from Super Mario Galaxy.
In Super Mario Galaxy, he is the king of the Topman tribe. In the first battle present in the mission "Topmaniac and the Topman Tribe", you need to jump on his head and defeat his minions and you have to jump on his head and push him into the lasers surrounding the fight. Once you do that three times he will explode and expose the Power Star.
In the second battle in the mission "Revenge of the Topman Tribe", you use the same steps, but he is a little harder to beat as he summons more minions and the electrical barrier you use to shock him is smaller.