Ability Points (AP) are needed to have your Guardian Forces (GF) learn their abilities. Check out the Ability Points (AP) tips and tricks section for more information.
Cactuar Island is the easiest way for your party to earn a lot of Ability Points (AP) very quickly. This will allow your Guardian Forces to learn the abilities that are available to them as quickly as possible.
You can access the island any time after you have acquired the airship, Ragnarok, but you can actually access the island much earlier than that using an easy trick close to the island. The earliest point that you can do this in the game is after you have gained access to the mobile Balamb Garden ship after completing the events at the Fishermans Horizon.
Complete the story events that take place at Fishermans Horizon and then jump into the mobile Balamb Garden ship.
This strategy guide contains a recommended side quest order with a few side quests coming before this one. Navigate to the Mobile Balamb Garden walkthrough page (by clicking on the link) for more information.
Specifically, you should make sure that you have used 
Amnesia Greens and 
Card Mod on specific cards in order to make sure that your Guardian Forces are learning the most optimal abilities.
Fly the mobile Balamb Garden down to the lower continents on the map which are collectively known as Centra. Find a beach that you can fly Balamb Garden up on to near to Cactuar Island (use the map below to locate the island).
Disembark from the ship close to the mountain range that has a small pathway running between it (shown in the screenshot below). Make your way across the Kashkabald Desert towards Cactuar Island and be sure to use Diablos’ 
Enc-None ability so that your party is not thrown into any random battles.
Check the menu screen as you get very close to the island and you will notice that the menu actually lists your location as “Cactuar Island” once you get close enough to it. This means that you can now fight Cactuars! The screenshot below can be used to help you pinpoint the location.
Navigate down to the section below labelled “Cactuar Battles” for more information on how to quickly gain AP from the Cactuars you will face in this area.
Reaching the island is much easier once you have access to the Ragnarok ship. Simply fly the ship to the island using the map below and jump out of the ship.
Be careful in this area though as you could be thrown into a battle against “Cactuar”. This is a Guardian Force that you can obtain but the battle against it is much more challenging than fighting regular Cactuar enemies. Check out the Cactuar side quest section for more information:
Cactuars have two little nuances that need to be dealt with. The first and foremost issue that you will run into when fighting these little buggers is that they have extremely high evasion.
Most of your party member’s standard attacks will miss. Squall, however, starts off the game with 255% hit stat (the highest possible) and it never changes. Squall will never miss an attack throughout the game. Use him predominantly when facing Cactuars.
You can also use any characters that have magic junctioned to 
Hit-J or use Selphie if she has her Strange Vision weapon which has 225% hit automatically.
The second nuance is that they like to run away. This means that you normally only get once chance to hit and defeat them before the battle is over. They only have 300 HP though which means as long as you can land a hit on them they will be defeated.
Each Cactuar you defeat will net you 20 Ability Points (AP) for each of your Guardian Forces. Stay in this area and grind out AP until your Guardian Forces have learned all of their most important abilities.
