So, a little cheat for hard mode, since you like using Yuffie and Aerith:
If you put the ATB Ward on Yuffie you can use her Brumal Form ability over-and-over again to charge the party's ATB.
For even more brokenness, give her the Keychain accessory that you get from Roche and cast Haste on her and you're never going to run out of ATB. It's broken.