Mega Man X Secrets:

Hadouken Fireball:

To access the fireball, first, you must find all the hearts, sub-tanks, and armor upgrades, and have all the weapons. Go through Armored Armadillo’s stage, and ride the last mining platform all the way until it jumps the big gap right before the boss’s door. Jump off of it right at its apex, and you grab the wall above the door. Climb up. If there is no capsule there, exit the stage and repeat the process. Note: you must have full health, full sub-tanks and most likely full weapon energy by the end of the stage to get the capsule, and it usually takes four or five run-throughs with the full life/weapon energy to get the capsule.

To activate the fireball, you must have full life. Press down, down forward, forward on the D-pad, then the fire button, and X will shoot a fireball. It will kill just about everything.