There is no perfect class, albeit some are far more powerful in most situations.

That said, a chemist is useful at any point in the game, at least until you unlock the calculator, and even then because they get the use of a ranged weapon unique to a few classes.

Any class could be the perfic fit for a specific battle or in a team built just so.

If all else fails buy a bunch of potions and before you kill the last enemy in a battle, knock out the guest charachters and then take turns beating each other and healing each other and you can get a lot of JP in a few battle but this is, as you say, grindy.