They only had to break through the elevator door and use the elevator shaft.

If they climbed that tunnel toward the heavens, they would reach the high-class apartments on the Dianoid’s upper level.

[...]

Hamazura’s Power Lifter was climbing the vertical elevator shaft by brute force. The steel machine had enough strength to achieve the speeds of a small car.

Toaru Majutsu no Index New Testament vol.12 ch.3 pt.9