If you reduce his HP to 0 during the fight and then either run away or blow him up you get 50 ap. The difference being that if you only reduce his HP to 0 he'll continue to chase you.
At the start of every fight (except for the first) it starts with 5 "lives". When you knock him down he looses a life, and starts displaying the repair messages. If you reduce his hp to 0 before it finishes repairing, it looses another life and stands back up at full hp. If it finishes repairing before you reduce it to 0, it'll stand up at full hp (but doesn't loose a life). When it's down to only 1 life it won't fall down anymore, and when you reduce it to 0 it'll lose the last life and blow up. (again the number of lives resets to 5 every time you fight it).
So if you want to farm it for ap, you want to reduce its HP to 0, then knock it down 1 more time and run away, you'll get the ap and it'll keep chasing you. --- [/rant] you just accidentally my whole signature
Note that you can only access the menu on certain screens during the escape. You may want to do a test run to figure out when those are so you don't end up wasting AP on worthless GH HP+ and Sum Mag+ abilities. I once defeated him 14 times for 700 AP, but honestly in the time I spent preparing to do that I probably could have gotten that much AP from regular enemies and progressed the plot a ways too. --- What senses do we lack that we cannot see and cannot hear another world all around us? -Dune Watch the Colbert Report! It's crumbelievable!
Not to mention that you get Diablos immediately afterwards and the amount of AP you need for him will give you most or all of the abilities for the previous 4 GFs.
Farming AP is not worth it. I just ran down to the beach, dodging him when I could, and destroyed him on the bridge. Fortunately I got no random battles after destroying him and only one coming out of the tower (I escaped) so I got back with about 23:58 on the timer. Although I didn't get top marks in every facet of my Seed ranking, I still got Rank 10. It jumped to rank 19 after Seifer's kills were factored in.
--- A fight is not won by one punch or kick. Either learn to endure or hire a bodyguard. Bruce Lee