I always just took a few detours with Yuna, switching between white and black magic as I felt necessary. I actually learned the T2 spells with yuna before lulu. I also spent some black magic spheres on her to avoid straying off the path of white magic too often. It's really useful to have some extra firepower for the turns where you don't need her to heal.

I also gacve steal to two extra characters, and gave use to almost everyone.