I never had any problems with that... probably not much of a comfort for you. :P
Use Tidus' Cheer ability (I think he starts with that?) which increases attack and defense. Then just keep attacking. When Rikku shows up, use her to throw all the Grenades you've got. When she runs out, steal some more.
Use Rikku for healing, not Tidus, because he has a stronger attack. Whatever you do, don't let one of them die!