OK so what I did was caught most of the monsters first, because the unlocks let you give auto-phoenix to some armours.
You want overdrive-AP on a weapon and I just went with double AP because again, it's easy to customise it with the items you get from the monster arena, and those items aren't used for anything else that's too useful.
Like Omega said, the triple AP/ Triple overdrive stacks (so you'd get x9 AP) but honestly, the time it takes to farm the stuff for that isn't worth it. I just did it for every character with overdrive-AP and double AP on their weapon. And I didn't really adhere to the normal strategy much either, I just had all 3 with comrade and all 3 attacking (when it was Tidus, Wakka and Auron, since they had a lot of kills) and that way they all get a shitload of AP rather than concentrating on one character. But characters like Yuna for example, you want the two with the most kills to attack and just skip her turn (for me that was Tidus and Wakka).
I usually got about 41 levels per battle, sometimes more. Really depends on pre-emptive/ambush etc. And both Tidus and Wakka had auto-phoenix and eventually they both had auto haste aswell. Auto haste lets you get something like twice as many hits in.
Tbh I don't like doing it, I find it super tedious to watch the karma animation over and over again. But I did that to get to a certain level (like so their attacks were hitting almost 99,999 with celestials, only takes about 110ish strength with most average enemies - talking about Tidus, Wakka and Auron only) and then instead of moving everyone around the grid that's when I started farming strength spheres, moved all the characters to the same point and started working my way round the grid, filling every node.
Those with double AP celestials kind of were the vanguards of the new grid because they gained levels faster while farming so the actual Don Tonberry battles you do after that point are far less. The problem comes when you have a character who hasn't moved at all or hasn't been used, then it's a pain in the hoop. However, for some of the monsters you have to farm for certain spheres, it's better to have a really weak character and have the monster go for them so it immediately fills the other's overdrive gauges.
tl;dr, focus on 2 or 3 characters and have one character (for me it was wakka) move round the grid and fill it with nodes/ clear bad nodes before you start doing it with everyone. You can get kind of stuck if you don't have a weak character to fill their overdrives with the combat mode. There are other ways to fill them easily (like using victor) but with earth eater the quickest way is to use multiple overdrives and victor only fills it when you win a battle, so it's kind of useless other than in specific situations.
Also, don't be put off by the task, it's very easy (and kind of relaxing). Like it takes half an hour - 45 minutes to farm all the defence spheres you need, and its the same for the rest. Easy peasy. And you won't have a problem with gil because they drop weapons and armour that are worth a shit load, just make sure you sell stuff when your inventory is full.
It's even easy to farm luck spheres. Takes slightly longer but it's not hard. What is soul destroying is the fortune spheres. Earth eater is the hardest to farm for any kind of sphere and it only gives you two. And you need 7 times the amount. I am never doing that again. It takes the same amount of time in total to get the number of fortune spheres you need as it does to collect every other kind of sphere for the board.