Shell - Reduces damage multiplier and hit rate of incoming magical attacks by half.

Shell [!White] - Always Hits.

White Mage

Buyable at Regole, Bal Castle, Quelb, Surgate, Moore for 3000 gil

Black Mage | Blue Mage | Freelancer | Mime | Red Mage | Summoner | Time Mage

World 3, Wonder Rod

Magic Shell [Passive Ability] - Only triggers when user hits low HP (Seems to be 1/8 Max HP in SNES/PS1 versions, 1/16 Max HP in GBA version).

Mystic Knight

Always in effect.
10 total AP. L1 ability. (Other jobs)

Mighty Guard [!Blue] - Always Hits. Also gives Float and Protect.

Blue Mage

World 3, Overworld (Sea, G3 on this map) - Stingray (Must be Confused/Controlled)
The Rift, Dimensional Castle (Boss) - Azulmagia (Must be Reflected)
The Rift, The Void (Boss) - Shinryu (Must be Reflected)

Beastmaster

The Rift, The Void - Crystelle

Dragon Defense [!Mix] - Always Hits. Also inflicts Reflect, Regen, and Protect.

Chemist

Mix Phoenix Down + Dragon Fang

Protect Potion [!Mix] - Always Hits. Also inflicts Protect.

Chemist

Mix Turtle Shell + Turtle Shell