I think Zell automatically has Armageddon Fist. That limit break isn't really called Armageddon Fist in the game, it just means you keep repeating two of Zell's attacks that are fastest to select, such as Punch Rush and Booya. For the record, I didn't spend much extra time on building up my characters on my first game of FFVIII. They weren't powerful at all, but they still defeated Adel with relative ease.