What is the best way to level up in ffxii?

This depends on what level you are when starting. A good place to train when under level 20 is in the Lhusu Mines in Bhujerba. Go to the second bridge and walk back and forth, killing all the skeletons that pop up, until you can walk across and back with nothing attacking you. Then run, without killing anything, to the first bridge and back. Repeat training on the second bridge. This will allow you to build up a huge chain, and the skeletons are good XP at this point. Level 20-30, stop on your way through the desert. One screen before the tomb, head north. In this area, you can go back and forth killing all the enemies, and once again, when nothing respawns, just leave, save, and come back. Between level 30-75, there is generally good training available just going place to place as the plot demands. If you need a little extra, do some of the harder hunts. Just make sure you don't skip any major areas without training, and your level should stay as high as you need it. Level 80-90, go through the Great Crystal in Giruvegan. Once you get to the top (you may have to draw yourself a map to not get lost on the way, and be a fairly good problem solver, or have a guidebook/ walkthrough) there are a couple of platforms with a nearly endless stream of Zombies. Just make sure you have Renew in your gambits before coming here. Level 70-90 alternative, go back to Lhusu. If you've kept up with hunts, you can enter the inmost shafts of the mine. Just kill the Abysteels, zombies, and skeletons that are everywhere, but make sure you have Libra and/or Float on a gambit, or the traps will kill you. Level 90+, the Henne Mines. If you've captured enough of the Espers, one of the warriors near the entrance of Jahara will open the inner parts of the Henne Mines for you. There are plenty of high-level monsters including dragons, ghosts, Abysteels, and reapers. Once you're in the high 90s, you can go capture the last Esper in this area.