Until you get some good players or techs, you will mostly rely on Tidus. What you should do is get Tidus and the ball as near as possible to the oposing goalie and then shot. If you did get the Jecht shot on the boat ride to Luca, it's much easyer, as you can knock out 2 blocking players with this tech, but the Sphere shot can be quite usefull in the begining too. Later on, as your players get some exp, you can mark oposing players of a lower level to learn theyr techs, and after that, the game will start being very easy.