Go to the following locations in San d'Oria when at the indicated levels to level up easily.

Level 1 to 7: West and East Ronfaure
Level 7 to 13: Ghelsaba Outpost
Level 14 to 18: Valkrum Dunes
Level 19 to 25: Peninsula
Level 26 to 34: Jugner Forest
Level 35 to 43: Davoi (possibly)
Level 44 to 52: Xarbard
Level 53 to 60: Deep King Ramperre's Tomb
Level 61 to 75: Ifrit's Caltron