X-Plasma is the combination of X-Ray Scope and Plasma Beam. This combination of items allows the player to exploit Phantoon, Botwoon, and Draygon to make short work of them. It is the trademark technique of GT Classic runs, as it is basically the only category where the player will have both items before encountering those three foes.
By activating X-Ray Scope as a charged beam shot containing Plasma passes through the boss's vulnerable point, it will reset the boss's invulnerability, causing the beam to strike again. This can be repeated multiple times per charged shot, effectively allowing all of these bosses to be slain by one charged shot, and making it quite similar to the infamous "pause glitch" in the original Mega Man.
Phantoon in particular will require being struck by a source other than the charged shot first before the exploit is performed. If Phantoon is struck by the beam first upon opening, he will immediately shut, preventing the exploit. A Missile works well as an initial damage source.
This exploit can also be performed on the steel pirates of Lower Norfair, but the window for doing so is extremely small.
This exploit can also be performed with Hyper Beam instead of Plasma Beam, because it shares the same penetration quality.