When you gain access to the MegaMan Buster, set it to a MiniBook Item(so you have a better chance of getting it). After getting it in the game, hold down the attack button for about a minute (until the light waves turn red). Then shoot, you should be able to get some one out of their transform state(because the shot is so strong).

NOTE: This gun can shoot four times, but when the Gun is fully charged it still has four shots, so dont worry about the gun energy.