You could also make sure the garment grids you use have a high MAG DEF and DEF and equip bracelets that add HP like Diamond Bracer and stuff.

Rikku seems better at Alchemist, Yuna and Paine as black knights and use Darkness each turn. Rikku on Mix Potion + Hi-Potion each turn, you should get him evantually. Don't bother with white magic, its too slow. Equip a Haste Bangle on Rikku so she can cast HASTE on the party so you can heal and attack faster than Yojimbo ;p