Before you start fighting the mushroom, make sure that Celica doesn't have a hat with eyes on her head. You can get rid of her and her annoying colleagues with an invisibility potion.
Celica should do everything herself. No Glace or Nel.
Before you start. Make sure that Celica has enough Celestian energy (at least 1/2). Otherwise, you need to meditate.
Turn on the opponent's health bar. This can be done with an artifact in your inventory or Celica's skill. Much depends on your experience.
The process itself. You can hit the mushroom with any spells up to about 50% of its health bar. After that, activate Celica's Celestian mode. Her spell list will change. Next, use the time stop spell (a round symbol with a figure in the center), and then finish it off with light spells.
