How to Complete Sleepless Nights
The objective in this battle is to defeat the Oversoul. The law forbids debuffs.
The Oversoul is an undead enemy, so use healing magic, potions, Phoenix Down, and any other undead-harming abilities that you have.
The Oversoul can bring dead enemy units back to life.
The Ghosts absorb Dark damage, and they can cast Dark spells, so they will use this to heal themselves and damage your units.