My favorite leveling/jp gaining trick involves Ramza's Yell ability and a Knight with speed break and power break. Kill all mobs on screen except for one which you surround, and then power break (this isn't totally necessary, just makes it easier in certain situations) and speed break. Then Ramza and the knight use a combination of Yell and Speed Break to give basically unlimited turns to whichever character you want. Accumulate is probably the best/easiest/fastest skill for this because it's self-targeting and is 100% accurate. Just make sure those characters that accumulate don't have some kind of counter equipped.