It is possible to get Ken and Ryu's Hyduken from Street Fighter II. With this Mega Man can fire a little blue fireball that kills any enemy in one shot. Too good to be true, huh? Well the catch is (Yep you knew it was coming) the shot only works when you have full life...Bummer!
First you need to have every heart container, every sub tank, every armor part, and every boss in the game beaten up to the Sigma stages. Then you have to go into the Armored Armadillo stage and play through it 5 times, and at the end of each time you HAVE to grap the life power up at the end of the stage (right above the boss's door). On the fifth time through the level you will see an armor capsule next to the life power up. Dr. Light will give you a "special present" the Haduken fireball.
You launch the fireball the same way you would with Ken or Ryu from street Fighter:
Down, Down/Forward, Forward, Y
Remember you can only launch the Hyduken with FULL LIFE!!