The Moon Bug can be quite an annoying monster to fight. Somewhat of a cross between the Rockanoff and the Pirate's Chariot, it rolls around fairly quickly, and can fire a huge cannon at great speed. Naturally, you'll want to avoid his shots by strafing sideways and closing in on him. Fortunately, while the Moon Bug will try to roll toward you, its turning system is not great, so side-stepping should be quite easy. You can attack as it rolls past you if you opt close range combat. Alternatively, it can't defend against projectile attacks, so you can keep strafing and firing projectiles at it until it dies.