Clash at Demonhead/Route 21

Description
Assuming you access this route from the left side, three small enemies will be bouncing along a bridge. Clear them away and proceed to the right. Watch out for sentry cannons and shoot each one you find as you make your way to the door on the right. One more bouncing enemy may appear when you reach the door.

Step inside the door, and proceed to the right, continuing to shoot each cannon that you encounter. When you get far enough to the right drop down and head left below the ledge. Fall through the gap to the screen below. Eliminate the cannons that you are able to shoot and ignore the rest as you drop to the floor and head to the right. Follow the floor all the way to the end of the room where you'll find a door on the lower level. Step inside the door to find the Governor.

Governor: Bopper
Before the fight begins, Bopper explains that headquarters is above your head (referring to ../Route 42/). Then Bopper attacks. Bopper travels diagonally around the room, but in a staircase fashion. Instead of moving diagonally, he alternates between moving a little vertically and then a little horizontally. He changes one of those directions whenever he hits a wall, floor, or ceiling. As he moves, he shoots a fireball from his vehicle. This fireball always advances, but homes in you vertically. You can shoot the fireball to nullify it at any time.

As you dodge or shoot his fireballs, shoot his vehicle as much as possible. Keep shooting until he is defeated and flies away. Using the regular gun is fine, if not a little tedious. Alternatively, you can use other weapons like Thundershot to make it easier to hit him from the ground, as well as to defeat him quicker. When he flies away, however, he does not leave a medallion behind. He's holding on to his for the next time you meet. Exit the door, watch out for the cannons, and jump up the platforms to the left. Continue to the right and jump up to the screen above. Defeat the humanoid enemies that approach you and head to the right exit.