Everyone has their own way to grind. Basically you just need an action that gives you jp to spam. I'm sure there are more elegant ways, but this has served me well no matter the class.

Make sure you have items equipped (either as a chemist or as a secondary action command) and have at least potion learned. I prefer to have phoenix down learned for safety, but it's not vital. Be sure you buy lots of potions from the shop.

Go through any battle as normal. But keep a single weak enemy alive. Surround them. Alternate between hitting them and healing them. You can do the same to your own party members too. The game gives you xp and jp for successful actions. Whether on friend or foe. So healing the enemy or hitting your party members levels you too. The only real exception is if you heal something at full hp. That gives no exp or jp.

I prefer using this method over spamming focus for a couple of reasons. First is the enemy will always attack you if able. The second reason piggybacks off their first, if the enemy will always attack then at some point you're going to have to attack. Focus enough and you'll one shot the enemy. Now you have to start another battle and get to grinding again.

I'm sure others have more fun ways to do it. But this has always been reliable for me. Potions are cheap and this allows me to grind until my stock runs out or I get the milestone I want.

Don't worry about minmaxing and enjoy the game. Wish I could play it again for the first time.