"[5 Turns] Makes the caster's skin as hard as oak wood. Greatly increases DEF."
Cost: 8 MP
Source: Equipping the Plant Core.
Harden Skin is a spell that applies three stages of DEF Up for five turns to the user.
This skill obviously proves useful on characters with smaller DEF, especially if you plan to cast Infernal Bloodlust on them. Alternatively, you can use it on Veronica along with Life-Stealing Touch to further increase her DEF during the effect of her Luring Flare, as having an already high base DEF will increase it to truly ridiculous levels.