I conserve leves for leveling crafting classes, where if I have a sufficient amount of them (>80) I can get one from lv1 to lv50 within a couple of days (though usually burning >200k gil in the process).

For combat subclasses I run the occasional dungeon and roulette, else I find somewhere I can grind without worrying so much about competition, while being able to farm mats that I either need for crafting or can sell for a reasonable amount of gil on the market board.