Chocobuckle, eh? Well, I haven't done nor explained this in years, so I might be a little off, but here we go: First, get into a fight with a chocobo whose level is a multiple of 4(use the Sense materia if you have it). Might want to have some greens with you so the little bugger won't run away. Then, cast Level 4...Suicide? I forget the name of the Enemy Skill, but it's the Level 4 one. Then you got it!