Here is a guide on how to access the secret Nova boss fight.
Hidden Nova Boss Location
1. Complete Sunrise Palace 1 on Demanding difficulty to unlock the Might Nova image pulse
2. Equip the above passive image pulse.
3. Turn on Story Mode + if you disabled it previously in order to have more dialogue in the upcoming Nova boss.
3. Select “World Moebius” stage, and before/while Moebius uses his “Damnatio Memoriae” special attack, switch to Gunvolt and intentionally get hit by the massive portal attack. You have to switch to Gunvolt for it to work.