>>533744265
Do as many hard chapters as possible.
Clear any chapter of enemies.
Maximize that Stage Clear count to upgrade your Outpost Level resource rate.
If you can't beat any more then open the yellow, red, blue cases in your item inventory.
Level up synchro.
You might break 300 and complete her story within a week.

Ultra is a BS boss, and the only way to defeat it is cover rebuilding to not get Nihilister 2.0'ed.