Mons Meg (Rebecca in the Japanese version) is a miniboss that appears in scene 4-1.
Depending on what version you play, Mons Meg is a robot but Rebecca is a doll. Mons Meg has a lot of castles on his head and ears and has a lots of cannons as well. He has a variety of meters and aim symbols on his body as well as a lot of colors (red, yellow,green, blue, etc.). Rebecca looks a lot more simplex than Mons Meg. She wears a green and yellow cute shirt with a green hat and a red ribbon in the back. She also has long brown hair and a red ribbon on her shirt.
Before you battle Mons Meg, make sure to stop at the right moment because Headcase appears to give you either a War Head,a Bomb Head, or a Hammer Head. If you keep on walking towards the miniboss and don't stop for Headcase, there's no way to turn back once you've gotten to the miniboss. Anyways, once you start battling Mons Meg, he will try to aim at you to shoot bullets (music notes if you're going against Rebecca) at you. He can also use his big fist to try and hurt you. Use Headdy to aim for his mouth which is his weakspot.