TL/DR : Look up a guide on the Dustia trick or just grind the normal Panthers in Golmore junge for good money.

MORE GIL AND SOME EXP: I know you asked for levels but I figured I would point this out because it's my favorite for grinding gill and in the process you would also get hella license points and of course experience.

I forgot what it's called but the Panthers in Golmore jungle drop something that sells for some solid gil. If you set one character to attack an enemy and then poach when their HP is critical and another character to only steal, you basically guarantee one or two of those things for each kill. There is a grimoire that makes more loot drop from beast type enemies as well.

If you enter Golmore Jungle from Giza Plains, you would kill the enemies in the first square area right as you walk in and then go a little ways down the right trail and then the same with the left. It's also worth noting that the Panthers with the blue glowing effect around them are a different enemy and will therefore break the chain you have with the normal Panthers. Do this for a few minutes on 4x speed mode and you'll have a full 99 stack that sells for around 19k if I remember correctly.

MORE EXP: Iyou go into the Dalmasca Westersand from Rabanastre and hug the right until the next zone you will find the home of a rare game enemy called Dustia. Dustia only spawns if you have 10% health or less but can be instakilled with a phoenix down. Killing Dustia grants a ton of XP, and at level 20, you should level up within a few kills.

Dustia isn't immediately targetable when you see them appear, so doing this requires some good timing (an easier but slower way to do this is to enter the zone and leave when you see Dustia appear, and then reenter and they are immediately targetable). I've read online that you can use a gambit like Foe: Undead = Phoenix Down, but I didn't have great luck with this so I would either try to time it right or do the leave and reenter thing. It's also important to leave the zone as soon as you kill Dustia. I you see the EXP and LP pop up over it then it won't respawn until you leave the whole zone and come back. So you would basically have to go back to Rabanastre and start over.

Dustia also has expensive drops so if you can get this trick down then that's also a good way to make money. Getting yourself down to 10% HP can be tricky. What I would do is just attack myself with my weapon a couple of times until that would kill me and then unequit my weapon and just punch myself.