Okay, so...
Get the Squire enough JP to get "Gained JP Up" and "Accumulate" Now switch to any class you want to level, and or get a great sum of JP in. Be sure to keep "Basic Skill" or "Guts"(for Ramza) as the secondary combat ability.
Go into an encounter battle, kill off all monsters except for one. I recommend doing this in Mandalia Plains, great chance of encountering chocobos, they are easy enough to get JP off of... Also, if you are going to do this, I highly recommend taking a knight with the skill "Power Break" for lowering the last monsters attack to 1, thus, only allowing the monster to hit you 1-2 or something around there... I recommend surrounding the monster, so when you are finished, just kill it (and if you have been "accumulating" for long enough, you can hit REALLY high damage to the last monster) And if you choose to pick the chocobo to surround, it will heal itself sometimes, thus healing every character around the chocobo... I do NOT recommend doing this on any map with zombies, unless you are skilled. LAWLz
Happy Hunting...
-Josh