Operation Babel: New Tokyo Legacy

Story related and cannot be missed.

Unlocks after completing the mission "7H, Haunted Hospital".

Once in the Haunted Hospital, you must defeat enough enemies before you can use the elevator to go down to B1F. Check the elevator every once in a while to see if you have defeated enough. Once you unlock the elevator, head to B1F X16 Y15, change the channel and there's an easy battle. Go to the elevator, and select 3F, then 1F, then 2F, then 3F again. After this, something will come loose in the elevator and you'll be able to reach 4F. Go through the portal and to the question mark to fight X-III Wizar and Walpurgis. Wizar likes to use poison attacks, Walpurgis has an attack that reduces your Unity by a lot. This should not be a difficult fight though.