This i what i did to get Tidus throught 3 Sphere sections:
1. Get the Ascalon
2. Train in Mt. Gagazet until the AP is worthless
3.Unlock the Caladbolg
4. Train in Omega Ruins
(I did this right after i got the Airship and i wasn't very strong)
It is also good to use a Return Sphere and go to the Spere Node right before Auron enters Tidus' section. That way, you can go backwords through Auron's section and become really strong, then continue through Wakka's section and become stronger. Hope this helped