From my experience, it's always the hardest story content you can tackle with 3 fodder + 1 of your guys, with a maxed supporter. Or 4 fodder + the maxed supporter but that can only handle up to a certain degree of difficulty depending on the unit. Max level units give 50% of the exp they receive to the units that are not max level. So the less maxed characters you have, the better.
But always do it with another thing in mind. For example, target farm certain catalysts and bring fodder with you. My Lorina I just maxed out needed 6 Banshee souls, and unlocking Yuna took 3 (and will take another 6 to max out.) So I was aiming for 9 Banshee souls to do the first two things at least. 4-9 World is the only place that drops it, and I farmed it with one fodder for a few days.
Stamina per XP wise, world story wins out since everything is only 8 stamina and it scales to 60+ level content. World difficulty event content (Diene) is fine too, especially when you need to farm the rest of the currency to get everything in the shop. But I wouldn't do it after you clear the shop, since it's 12 stamina each run.
