Barret's Mindblow definitely comes in handy against Rapps. I always go into the fight with Cloud and Barret's Limit Breaks ready to fire too. You can buy items in Wutai that give you Fire 3 and Bolt 3 to use against him instead of your materia. It shouldn't take too long if you have those.

I don't tend to worry too much about certain characters being under-levelled. They level up reasonably well on their own anyway. I do level up Yuffie before Wutai though so I can leave there with Leviathan in tow.