I should clarify.

WIS doesn't protect you against offensive magic, it protects you against spells like Charm, Dominate Person, Fear, Despair or other spells or abilities that mess with your head and your will/resolve.

Unless you have a particular weakness to a certain element or type of magic, nobody is more or less susceptible to being blasted by it. A person with 'F' WIS will take as much damage from a fireball to the face as someone with 'A'-rank WIS, provided neither have weaknesses or resistances to Fire.