Island Closest to Hell/Heaven
The Island Closest to Heaven and the Island Closest to Hell are two islands that can only be reached once you have access to the Ragnarok. On these islands, you can find a multitude of high level monsters to fight as well as some invisible magic Draw Points scattered throughout each of the islands.
These two islands are great for amassing both EXP and high level magics, although in a typical playthrough, most gamers will not need to venture to either of these islands, so this is definitely an optional stop.
Keep in mind though that if you’re searching for Ability Points (AP) for leveling up your Guardian Forces that the best way to do this is using the Earning Fast AP Trick using Cactuar Island.
The two islands are the farthest west and the farthest east island on the map; more specifically, the Island Closest to Hell is the one on the west side of the map and the Island Closest to Heaven is located on the east side of the map.
That being said, because the world map is a sphere, you can travel between the islands fairly quickly by flying over the ocean. Pull open the menu screen to make sure that you’re on the right island.
Island Closest to Heaven
Enemies in this area include Chimeras, Malboros, Ochus and Tri-Faces. You can also Draw the following magic on the world map. Just rapidly press 
as you walk around the map.
AuraThe enemies on the Island Closest to Heaven are slightly easier than those found on the Island Closest to Hell.
Island Closest to Hell
Enemies in this area include Grendels, Ruby Dragons, T-Rexaurs, Hexadragons, Blue Dragons and Malboros. Again, you can Draw more magic as you walk around the world map. Just rapidly press as you navigate around the island. You can also use 
Enc-None to just search out the various Draw Points.
Note that the Island Closest to Hell has the greater concentration of high level magic Draw Points and more difficult enemies.