What I did to make sure everyone was getting EXP even when grinding was put in the party I didn't use (Auron, Kimarhi, Rikku & eventually Lulu) to begin the battle and then switch them out to my normal party which was Tidus, Yuna, and Wakka. But Wakka and Auron were interchangeable in my group so those two switched a lot. That way everyone get points for each battle. I grinded away in that cavern...was it Zeritan Caves? Where Omega Weapon is