Yea, I personally just came back to my old save file and ran into the same problem.
There isn't really a guide for unlocking all the overdrive modes, especially some of the more esoteric ones for lesser used characters.
Here's my recommendation: 1) I wasn't looking to unlock the status ailment ones or anything, so I can't help you there, but I needed to unlock comrade for some completely unused characters and loner for fortune sphere farming
2) You CANNOT unlock overdrive modes in Monster Arena battles. I suggest Omega Ruins for overdrives that depend on the enemy fiends doing something, simply because higher level mobs will have a higher Agility, and pressing triangle 10 million times gets old fast.
3) If you still have multiple characters with low agility, use them. If you can, get them all armor that has AUTO-PHOENIX (very good for this) but NOT auto-haste (very bad for this).
4) Cast Haste on the enemy that you want to attack you or your party.
5) You can try to cast slow on yourself, but I don't know if it was because all my armor with auto-phoenix also had auto-haste, but I couldn't land any type of Slow or Slowga on people that have auto-haste on their armor.
6) Spam triangle until you run out of phoenix downs. They only cost 100 gil each or something silly.
7) I don't think you need potions (for auto-potion), just letting my guys die and be resurrected is cheaper and faster (assuming you're not getting 1 shot).
8) Good luck, this works especially well for stuff like comrade (for the AP trick).
Getting things like Daredevil, Ally, or Loner is much easier. Just kill off all but one mob, cast slow on it, make sure you have haste (or auto-haste) on yourself, auto-life if you're doing loner, and spam that triangle.
(Absolutely no auto-potion for Daredevil, obviously!)
Fighting a low level mob would be slightly easier for Loner, et al. I still used the Omega Ruins just because it's easy to get to from the air ship, and I was already there for the others, so it doesn't matter too much (perhaps a Calm Lands mob while on your way to the monster arena?).
Fight an Element or a Flan. For this example, I'll be trying to get Daredevil on the three girls with Lightningproof armour.
I'll go to the Thunder Plains and fight the enimies there half heartedly until my HP is all in the yellow. (If it helps, use a Stamina Tonic)
Now I'll fight against a Gold Element. I'll get rid of the Buer and the Iron Giant there because they'll screw this whole strategy.
Now I'll cast Hastega and Slow and the Element will just keep casting Thundera. Because I've put a Lightningproof armour on the girls it'll do jack. Because I'm doing Daredevil, Thunder Eater will just put my HP back up.
I have a turbo controller, so I'll just turbo the Triangle button and go do something else.
When I come back, Yuna, Lulu and Rikku all have Daredevil.
If you don't have a turbo controller, you'll have to sit there and press the Triangle button hundreds of times until you get it.