X-potions are useless later on until you have around 700+ MP. I would set Curaja gambit for only one person in a party of three, also Hastega for one person, and Esunaga. The essentials are obviously the usual buffs, such as; Bravery, Bubble, Protect and Shell (I never really have to use the last two) Best leveling up spot is the Pithead Junction in the Henne Mines Extension area - 20 bats will appear, one kill = 3100 EXP x 2 w/embroided Tippet = 6200 per kill x 20 = 124,000 EXP, it's easy to shift screens to get them to respawn, with no encounters from other monsters, exit only twice then comeback. Only need three gambits, four if necessary; Foe- Undead : Cast Sleepga ---- Foe- Sleep : Cast Curaja
----- Ally - Cast Bubble (Before you pull the lever and haste up party, also definitely make sure you cast Faith on everyone in party)

Takes around one hour to level up 10 levels at lvl - 77+, but still good leveling.