Did you complete the Nirvana? It's a shame if you did, because Belegmine(?) is a sphere (insert the bad B word). You can crop SO many spheres of many kinds from her and her weak aeons. But, yeah, the Arena may be harder, but you get more of them. Max is twelve in defeating The Magus Sisters in Remiem Temple. But yeah, if you haven't yet, that's a very easy way.
You don't NEED to max Luck..unless you favor Rikku (<3) and easier critical hits.