Yeah....grinding in chapter 4 is kind of pointless (if you can avoid it) as you can get one round random encounters with 500+ JP (999 with growth egg) in chapter 5. We need to get you up to Chapter 5 as soon as possible. So at this point the questions become, what level are you and which boss is giving you trouble? The advice for what to do depends on which boss is giving you trouble (as I recall there are 5 bosses with additionally two optional ones and one sidequest that has 7 bosses).
hold up, what's the trick there.
Not really a trick. Just have to be aware of the fact that Rafflesias give a ridiculous amount of JP. Plus you can find them in groups of 6. You find them on the Water Temple Continent. I think the encounter rates are the same in the dungeons and the world map, although I think I tended to use one of the dungeons. When I was level 60ish I was able to one round them by spamming 4x Crescent Moon with each character. Just set that to autobattle run around and you should be able to beat everything in the area and do light speed grinding. One thing to remember is to grab the accessory that prevents enemy first strikes. There is one enemy encounter in the area that can f you up if they get the first strike.
Not to mention the fact that completing sidequests in Chapter 5 and beyond will net you 999 JP guaranteed (and almost all of the sidequests consists of a single battle), and there are a ton of them.Honestly? I found those to be a waste of time (for grinding) because I had to be in a lower level class to benefit which made the battles take longer, and it was just a lot faster to beat up the random encounters for JP.
When I started grinding for jp/exp in chapter 6, I just hung out in the earth temple, and abused the hell out of crescent moon and dualwielding. It's easiest in chapter 6 after you get access to the sage class, as they learn an ability called obliterate which 1 shots enemies at the start of a fight so long as you don't get surprised. Full exp, jp, and pg, so it's absolute cale for grinding.Obliterate does make things go faster. Using Crescent Moon on Water Temple Continent random encounters in Chapter 5 is fast enough though. Or at the very least it works for getting you the 20 levels you need for Obliterate to work.