At this point, the AP trick is up to you. Every time you press triangle
(defend), you'll accrue a lot of AP. A weak enemy like Dingo won't
substantially hurt you the few times he'll have a chance to hit you,
so don't worry about your HP. Just keep hitting triangle to defend.
Personally I use my old PSOne turbo controller and tape down the
triangle button. Anyone who remembers the FF3 waterfall leveling up
trick with Banon will remember doing something similar. If you don't
have a turbo controller for PS1 or PS2, then it is up to you to manually
hit triangle over and over again. Regardless, the amount of time you
leave your controller doing it or you yourself do it is arbitrary. I'd
say to do it for about 10 minutes at first and see where you end up
since APx2 or x3 can really speed this whole thing up.
After you decide you've had enough, go ahead and kill the enemy. You'll
see your AP is a ridiculous amount and surely you'll get to S. Level 99.
Most likely it will take you a few times of doing this to get around the
entire Sphere Board.
Problems:
The biggest problem you'll encounter is running out of regular sphere
board pieces. You'll be so powerful that you might as well fight several
random battles in any place you feel will yield the most.
The second problem you'll run into is that you'll be at 9xxx HP and
you'll still have more than half the board left. I suggest consulting
DokiWaku's excellent Secrets guide. You'll find out how to exceed Max
HP (up to 99,999) and exceed Max Attack (up to 99,999), etc. Just a note
though, some of the ultimate weapons have these properties intrinsically.
For example, Tidus' ultimate weapon already has the exceed Max Attack
attribute.
There's a good chance that Yuna and Lulu won't have the Alone
overdrive. To teach them it, enter into a battle with a weak
enemy and kill off everyone else in your party. Obviously you can
only do this with one character that needs the overdrive method at a time.
Then just sit there and keep hitting triangle over and over again.
Cast curative and defensive spells accordingly. It will take a
few minutes of doing this so be patient. Eventually a message
will show up telling you that you've learned the overdrive
technique 'alone.'
That pretty much covers it.