Cactuar Island is the easiest way to gain a lot of AP fast so that all of your Guardian Forces can learn the abilities that are available to them. Even if you cannot actually make it on to the island yet (due to not having the Ragnarok) there is a way to fight a few Cactuars to start racking up that AP.
If you do have the Ragnarok though just fly over to the tiny little island identified in the map at the top right of the screen (bottom-right corner of the map). Otherwise, if you are stuck getting around with Balamb Garden continue following these instructions...
The Island can be found among the lower continents on the furthermost east side them all. Use the screenshot of the map to pinpoint the location.
Swim Balamb Garden up close to the island. When you reach the island look to the west and you will see a beach (it’s the beach shown in the screenshot to the right with Balamb Garden parked on it). Take Balamb Garden up on to that beach and then disembark and walk through the narrow causeway. Make your way across the Kashkabald Desert towards Cactuar Island once you get through and use Diablos’ “Enc-None" ability so you can avoid random encounters.
Check your menu as you get close to the Island and the menu will actually list you as being on Cactuar Island once you get close to it, which means you can now fight Cactuars! For example, in the screenshot shown to the right, Squall is currently listed as being on Cactuar Island.
Time to start fighting some Cactuars...
Cactuars have two little nuances that need to be dealt with. The first and foremost is that they have extremely high evasion. Most attacks made against the Cactuars will miss, however Squall begins the game with 255% Hit stat. This is the highest amount attainable, which means that right from the beginning of the game Squall cannot miss with an attack. Use Squall for all of your attacks.
Using Squall also helps to address the second of nuance regarding Cactuar fights, and that is that they like to run away.
Each Cactuar killed nets 20 AP points for each GF, so you can fight Cactuars until you learn all of your Guardian Forces most important and useful moves. Cactuars have approximately 300 HP so as long as you make contact you will easily defeat them.