I know this question wasn't directed at me, but I thought I'd quote it with a response since your post was buried on the previous page.
I've been focusing on my low-level characters one at a time--usually whenever I wind up with enough rupees to train them up from around level 15 to 35. (Seems to cost around 350,000?) At 35, I'll use that character to collect any heart pieces or Adventure items until they hit 39-40. I stick to +Bonus/Weapon Drop apothecary potions to increase my odds of grabbing a +EXP weapon along the way. (I also prioritize +damage to weak gauge and +speed for taking camps in the badge market.)
At 40, they're usually capable of soloing the "True Partners" map, so I run them through that until they get to be around level 50. Seems like this stage is your go-to option for straight grinding. At level 50 I tend to go after Lv. 3 weapons instead, and I'll usually wind up with a few more levels in the process of retrying.
There are probably faster methods, but this has been my process. About half of my roster is sitting between level 50-70 as a result. I think the most important thing is to change up whatever you're doing so that it doesn't become super monotonous.