Use one fire, and as many of the same coin as possible to receive as high an XP bonus cast as you can. Do this for all different types of coins you have. Equip one of them. Go north of Hammerhead to the crater, or to any other place with an enemy/enemies that will serve for a pretty long fight. Cast all of the spells at the enemy, try to make fight last long enough to get them all off, then kill the enemy/enemies when you've run out of spells.
It helps if you equip bad gear on your allies, and unequip as much as possible.
Also, obviously go to Altissa before you start to get the 100% more XP food, and after to get the 3x XP sleep bonus.
You don't want high potency though, that just increases damage dealt. I did forget to mention Moogle Charms though.