JP Farming in this game is very tedious, even with efficient methods.

That aside, the method is similar to Archita idea. I haven't cleared Chapter 3 despite 60+ hours on the game, there may be better efficient method in later chapters. But this is doable once you have the Petal Gauge at the top left screen and access to Ooe Mountain.

However, the preparation will take a lot of time and requires a lot of button mashing.

Update: Did a quick look at Ritualist skilltree, the method remains the same.
Look at Method for quick xp method (aim for very high level characters, mostly level 60+)

There's several requirements to make JP farming efficient, listed below:

Reduce MP Cost at Level 3
Aome with Dragon Vein at Level 4
3 Maiden
1 Support Class that have AoE heal / buff (Example: Shintoist with Healing Ritual or another Maiden with Divine Heal / Divine Sake Dance)
Every Person must have Proficiency+ at Level 3

Getting Reduce MP Cost:
You must get Magician at Level 6, to unlock Spiritualist. Get Spiritualist to level 8 to unlock Celestial, that's where you get Reduce MP Cost passive skill.

Getting Maiden:
Get Priest to Level 6, to unlock Shintoist. Train Shintoist to Level 8 to unlock Maiden (For female, Male will get Ritualist).


Optional: (Strongly Recommended)
In Option, turn Skip Scene, Fast-Forward & Remember Cursor Position to "Yes" to save a lot of time.

Make sure your Maiden learn Divine Healing / Divine Sake Dance and S-Divinity Dance asap. You must have S-Diviniity Dance at max level, Divine Healing can be left at level 1, unless you want to level up the skills.

Make sure you get Reduce MP Cost at Level 3, this will allow you to spam Divine Healing / Divine Sake Dance per turn. Having MP Recovery Passive skill also helps a lot, you can find MP Recovery Passive skill in Incantor Skill tree, you will need your character to have Magician level 6 to unlock that class though.

Have Aome with Dragon Vein at level 4, ensures Maidens to use Divine Healing per turn (probably giving more MP than necessary).

Later on, you will have to move Maiden at side Job if you want to level other classes (Or have it on Main Job if you need to level them up)

Once you're prepared, go to Ooe Mountain, Select Request and choose "Clear Sealed Pit".

-Clear all Mobs
-Destroy All Rocks except one Rock (Or else, you will end the quest)

Go to this ideal Spot, show on this pic:

Have 2-3 Maiden start using Divine Healing / Divine Sake Dance at every turn, the optional class that's not Maiden must use AoE buff / heal / utility.

Aome must spam Dragon Vein per turn, unless you want her to boost the gauge as well, then you must manually select Dragon Vein with other Skill that AoE Heals / Buffs / Utility. I highly advise just to have her spam Dragon Vein to save a lot of time and hassle.

Have 1 Maiden to spam S-Divnity Dance at every turn, this character who's spamming S-Divinity Dance is your primary target for JP Farming. (This only works on Females, you will have to use AoE Heal / Buff / Utility for males)

This Method is pretty much unlimited use of 1 Petal, but the requirement to do so... Is a lot of patience.

You will get 200 - 250 JP (or around there, this game is so inconsistent. For males, depending which characters it can be ranged from 100 - 250) per turn after the first initial use of S-Divinity Dance in a battle.


XP Method:
Very similar to JP Farm, you will have to weave S-Spirit Weave (Ritualist special ability) with S-Divinity Dance to upkeep JP + XP buff on each ally.
If you will farming JP while buffing / healing allies, you would have notice that at very higher level will give you +15 XP when using AoE buff / heal / utility, with S-Spirit Weave. You will get +30 XP instead, can't really think other ways to gain more XP due to how bad XP gain is in this game.

You might want to watch videos or listen to music while u doing this. It will take a long time to farm 20k+ JP.

Can't really think a way to gain more XP at very high level, I happened to focus too much on JP to the point not noticing several of my characters were already capped.

Also the Cap for JP is 99,999.