Elevator I is a skip done by Nero in Mission 9 to avoid going through the first gauntlet where he fight scarecrows. Vergil can immediately jump into last elevator since Aerial Rave B and Trick up grants so much height.
General Info
To perform this skip you need have DT available, Exceed 3 and Calibur (If Console or 60 FPS)
There are plenty of variation for this skip. The main thing to note that some of them are exclusive to PC only. Because 120 FPS gives any characters extra height when jumping and such.
The timing of CS into Wall Hike is the key for succeeding this skip. It's always better to check the first two Roulette Spin before doing the final step.
It's possible to perform this skip without DT but it is not advised.