Earn Easy AP:
Walk around in the darkness for awhile in the field and forests around the town that has the auction house and lady who collects the zodiac signs. You will encounter a ghost. Give it some ore and you will win the battle, resulting in 10 AP.
                                                     

OR

Go to the forest west of the mountain path when going to the Lifa Tree. It is the only forest there. Walk around in the forest until you encounter a friendly monster called "Ladybug". Give it some Ore and it will reward you with 30 AP.

Earn Easy experience:
Use Quina's level 5 Death spell against the level 60 Grand Dragons on the outside of Gizamalike's Grotto for around 8800 experience per person. Equip the quick level gain support ability (level up) for even more. Also, remember to take Coral Rings to protect against the Dragon's Thundaga spell.

Find a Dracozombie. In Disc 4, you can find then on the small green patches of land near the Ipsen's Castle on the world map. You must have Zidane, Dagger or Eiko, Amarant, or Quina in the battle against Dracozombie (you can use others character if you needed). Have Dagger or Eiko use the Silence ability on Dracozombie, because it can cast the Lv 5 Death spell. Keep defending until all your characters are affected with the Zombie spell, then kill it. Use Magic Tag to cure the character that you want to level up quickly. Equip the Quick Level Gain support ability (Level Up) to make your character get more experience. You must disable the Auto-Regen and Auto-potion support ability on every character which is affected by the Zombie spell. You cannot use items to cure or use spell such as Cure, Cura, Curaga and White Wind on the affected character. Amarant's Chakra skill is essential, since it still works on the affected character.

When you finish beating the plant brain, right after you go out the door stay in the same location. Do not move and the creatures well keep appearing. They are easy to defeat and give a decent amount of EXP and AP.

Fight the monster called Yan or Chimera. When you fight this monster, keep defending until it drops or bites your party member and gives them viruses. Cure the person that you want to level up. Then fight a monster that has a lot of experience such as Grand Dragon, Iron Man, Vepal or Worm Hydra. Fight until the party member that was cured from the virus levels up.

Once you are on Disc 4 and have the Invincible Airship or a Gold Chocobo, go to the island southwest of the Iifa Tree. It is a long, thin island with a mountain and a forest. On this island (called Vile Island), you will fight enemies called Yan in mostly groups of three. These enemies are extremely difficult, but each group of three will give each character 28,000 EXP (more if a character is "snorted" off the screen or killed). To survive these battles, try to have all your characters equipped with Auto-Reflect, Auto-Haste, and make their Magic-Defense as high as possible. Note: If these creatures "snort" all your characters off the screen, the game will end. Make sure you have a lot of Tents, Vaccines (to counteract Virus Powder), and save between battles. Note: This is the same island to find the final friendly monster, the Friendly Yan, in the forest.