This is unlocked on Level 7 Haunted Forest.
These ghosts can only be killed with the sunray spell, you get this from the shrine in the graveyard area.
You can find the shrine with the detect kuji magic spell and follow the butterflies.
Once you have obtained it, use the sunray spell on the ghosts, they drop blue orbs which will refill your stamina bar so you can keep using it.
Defeat all the ghosts (there is about 10-15) and the trophy will unlock!