So, after screwing around with FFVI's memory... I've found a way to make Fiend always do his Fiendish Rage, no matter what. I've discovered it while trying to find a way to have player characters do an enemy's Special Attack, in this case, it was with Fiend's Fiendish Rage... but so far, I've not any luck. Knock yourself out; he'll always do Fiendish Rage.
Side-effects: You can't attack; instead "The entrance is blocked by an unseen force." will pop up if you try.
If anyone can find a way to make Special Attacks like Fiendish Rage accessible to Player Characters without having to use Rage, Sketch, etc., let me know. Oh, and if it isn't obvious, the code only works during battle. If you try it with other foes, other than Fiend, they won't do Fiendish Rage.
Code:
Fiend Always does Fiendish Rage 82003412 0006 82003504 012F 82003506 012F 82003508 0B30 8200350A 0B30 8200350C 0B30 8200350E 8821 82003510 0C2B 82003512 0C2B 82003514 032E 82003516 032E 82003518 0222 8200351A 0222 8200351C 0222 8200351E EF00
If anyone can find a way to make Special Attacks like Fiendish Rage accessible to Player Characters without having to use Rage, Sketch, etc., let me know. Oh, and if it isn't obvious, the code only works during battle. If you try it with other foes, other than Fiend, they won't do Fiendish Rage.