my advice are :

1. don't overlevel too much, because the enemies at story battle are at fixed level, so if your level too high, you'll be beating their ass without any resistance at all. My point is, there're no challenge if you're overleveled. Because FFT vanilla challenge lies on the first, second, and half of the third chapter. At the fourth chapter, i'm sure your level would be much higher than the story liner, for example my ramza at 78 while the last boss are no more than 50 cmiiw.

2. Define best job, the best job to kill multiple enemies at once is clearly calculator with their math skill. But keep in mind that calculator stats growth rate are horrible, so you wouldn't want spending lv on calculator, my advice is you should master calculator without gaining lv if possible.

3.About JP grinding, rather than wasting auto potion, i suggest you learn HP restore from Monk. this way you can hit your own party member without wasting any potion. just make sure the one you hit doesn't have COUNTER or any avoid abilities. and YELL is a must on grinding, go figure.

4.And another important thing, you should have at least 1 person with steal abilities at all battle. Better if you have 2, because there're a lot of battle which the enemies having great gear, not to mention (SPOILER)the rare perfume Chantage, Barette, Cacusha, Masamune, Genji set. good luck on getting the genji set from elmdor.

please consider my number 1 advice if you want this game to at least a little challenging. In my opinion, if you're not overleveling and not underleveling, chapter 1,2,3 difficulty are about 7/10, and the other chapter are about 4/10... so if you're overleveled, you wouldn't even have 7/10, maybe 5/10.