Bartz is probably my main (along with Terra), so I can give you a bit of advice.
Bartz's strength lies in his flexibility. His attacks take a little getting used to, but have a lot of utility when you figure out their strengths. Most of his moves, being a fusion of two other characters moves, have a combination of effects. Take Slidehazzard. It has the mid-to-close movement of Tidus's Full Slide, and the downward Wall Rush of Climhazzard. Reel Impulse breaks guards, draws the enemy in, and then knocks them away.