Earth Eater farming is the worst part, since you cant Distill Luck, and the "best" method is kinda monotonous (I'd rather mash A).
Loner Tidus -> Slice and Dice (x3-4)
All for 2 spheres, times 3 persons... roughly 60 spheres or so total (30 battles - thats if you dont get a Dark Matter instead).
I'm putting my Luck around 120, thats supposed to be enough to hit Dark Yojimbo consistently (even with Luck Skill x 5).