Kiriwo Terrible

From MangaHelpers:

There are huge monsters wreaking havoc in Mikagura Kiriwo's city. Building on his father's work, Kiriwo has created a serum that turns him into a giant monster for a short time. He uses this serum to help protect the town from a monster one day--only to find that everyone thinks he's an enemy, too! Everyone at school and in the media hates the "black monster" that he became, due to the massive destruction he did to the town around him. A girl from his class even has to quit school, due to problems caused by the "black monster." Will Kiriwo be able to show that his monster form is really intended to be a savior? [tethysdust]