To get Ryu's Fireball get everything (including defeating Sigma's robots such as Spark Mandrill),that also means all 4 energy tanks and all 8 heart pieces and all special equipment from Dr. Light. After you get everything go through Armored Armadillo's level 4-6 times without dying. When you get to the end of the level climb up the wall and get the big energy then go to where you fight Armored Armadillo. You will exit the level but go back to the level 4-6 times doing the same thing without dying.