Mega Man Powered Up/Proto Eye

Boss: Proto Eye
Stun weapons: Mega Buster (charged), Proto Strike

Proto eye is pretty much the easiest boss battle in the game which all characters are going to be able to defeat it easily.

As it comes toward you keep attacking until it gets close to you. to get behind it, be ready and as it jumps again run under it and get to the other side.

In normal mode each time that Proto eye gets to either side of the screen, it'll shoot a green ball at you. Which is really easy to dodge.

In hard mode it'll shoot three balls. It's better for your character to stay on the other side of the screen and try to jump above the one in the middle (short jump to avoid hitting the highest one.)