I personally used the inside of the grand marshals castle in eternia, right next to the save adventurer.
with auto fight, 4x speed 200% encounter etc and with 4000 exp, 4000 pg and 300 jp it was easy to get enough for whatever i needed, i'm sure i overlevelled too, level 70 at the end of chapter 5 didn't sound right
i have found the ultimate best spot to grind in chapt 5 ever but you need a few things:
lvl 5 arcanist with twilight black mage lvl 14 growth egg x1 (lvl the accessory shop in the town) cloths pin x3 (you get this from the fox at the old mans house) wind abate support skill (get this from thief)
here's what you do:
go to the cave that leads to mt fragmentum on the west side that's the side where florem is. do not go into the cave instead stay around the entrance.
equip 1 character (not the arcanist or black magic user) with the growth egg equip the other 3 with the clothespins
put the wind abate skill on everyone (if you can) and equip everyone for defense that means put all defense skills up, raise defense, shield lore anything that makes you a tank cos all these sob hit hard even at lvl 60+
give your arcanist the black magic 2nd job and give them the group cast all ability the rest put defense up, shield lore etc.
crank up the encounter rate and get ready
you get several enemies here trees, mushroom, birds and little tree guys all give pretty good jp and exp...but sooner or later you will get 6 of them little tree guys THIS IS THE HONEY POT!
the birds hit you with wind magic that's what the wind resistence is for. and them damn trees try to put you to sleep if they do than kiss your ass goodbye! that's what the clothespins are for.
fighting all these is pretty simple just default everyone except the arcanist. than brave once and use sleep on all enemies than use twilight. this will put all the enemies to sleep and they all fall for it here every time. than twilight wipes them out instantly. you'll get the one turn hero, sweeper and no damage bonuses every time and get (depending what you fight) 400-700 jp. but that's not the best part. remember them 6 tree guys if you wipe them all out in 1 shot you will get 999 jp and over 9000!!!!!!!!!!!!! exp!!!!!!!!!
stay here as long as you like i must have gotten 15 levels in an hour at least and still going! now and than you will probably die so save often and also the guy who has the growth egg will die from twilight if them trees put him to sleep LOL don't worry just revive him after the battle and heal. :P