Originally Posted by
http://www.playonline.com/pcd/update/ff11us/20060725vNusN1/detail.html
An accumulative magic resistance effect has been implemented for monsters that /check as "impossible to gauge."
Affected Magic Types
The following magic types will trigger the accumulative magic resistance effect when used repeatedly on monsters whose strength is "impossible to gauge":
Black magic (elemental)
White magic (divine)
Ninjutsu
Magical Blood Pact abilities
Magical blue magic
Magical weapon skills (Spirits Within, Red Lotus, etc.)
Certain job abilities (Chi Blast, Mijin Gakure, etc.)
Resistance Fluctuation
When a monster with accumulative magic resistance is hit by a magic attack, the amount of damage it receives will decrease in accordance with that monster's resistance. At the same time, its resistance will increase.
The monster's resistance will increase in accordance with the amount of damage incurred, but each monster has a maximum resistance level.