Capcom decided to make some easy money, so they recycled parts of Mega Man X5, threw in overly difficult trial-and-error gameplay, made almost everything an obstacle course, and called it Mega Man X6.
This run uses both X and Zero. Zero's sword physics makes it the ideal boss killer but some bosses have unavoidable invincibility periods, which is why X is sometimes used. The run also obtains a weapon which is more beneficial to X than to Zero. Zero cannot be obtained right away, but must be obtained by beating a certain boss.
The run uses the hardest difficulty level, Xtreme, which would be more appropriately called insane.