I've noticed that many FF XII players are wondering
" what is the best way to level up? "

besides fighting strong monsers, there is an Accessory
named - Embroidered Tippet - it can Doubles EXP Earned in Battles
you can find it in Old Archades-Tchitas Uplands.

this is what I came up with from discussing leveling up in FF XII with a friend of mine who is FF series hilarious

PS: better to exchage the Accessory between the party.

Good-luck in leveling up !