The Islands of Heaven and Hell are two unmarked islands in the world that happen to be brimming full of magic and dangerous beasts. First, however, you'll have to find them. The Island Closest to Heaven is just off the Estharan continent, northeast of the Chocobo Forest in the Grandidi Forest - the entire peninsula nearby is covered with forests, so it should be pretty easy to spot. The Island Closest to Hell is a jagged island, the furthest to the west on the map off the Galbadian continent. If you check your menu while on the islands they'll be properly named, giving you a clear indication that you're in the right place. Otherwise, use the guide's main map to help find these areas, along with the provided screenshots.
As mentioned earlier, both islands have numerous draw points containing high-level spells, but are also populated by powerful foes. On the Island Closest to Heaven is populated by Chimaeras, Malboros, Ochus and Tri-Faces while the Island Closest to Hell is infested with Blue Dragons, Grendel, Hexadragons, Malboros, Ruby Dragons and T-Rexaurs. All enemies on these islands are level 100 regardless of your party's average level, making them very, very dangerous... and lucrative. Malboros are probably your single greatest threat due to their Bad Breath attack, and you could find them quite difficult without the combination of the Initiative ability and specific magics junctioned to Status Defense (Berserk, Confuse, Pain and Sleep)... or the Ribbon ability, if you have Chocobo World. Ideally, however, you'll use Enc-None to avoid the monsters entirely so you can focus on harvesting spells, at least for the time being.
Again, check out the guide's main map to precisely locate the numerous draw points on these islands, but being specific isn't really necessary - with Enc-None you can just run around the perimeter of each island mashing [TALK/CONFIRM] until you find a draw point, slurp up its delicious spells, and continue. When you've circled the island, pay some attention to the interior, then fly off to the next one and repeat. These islands are mentioned first during this grind for two reasons: 1) because having high-tier magic will make everything else easier, and 2) because you can - and should - return to these islands regularly as you go about other business to siphon spells.
The draw points on the Island Closest to Heaven and Hell include:
|Island Closest to Heaven||Island Closest to Hell|
|Aura (x3)||Aura (x4)|
|Flare (x3)||Flare (x3)|
|Full-Life (x3)||Full-Life (x4)|
|Holy (x3)||Holy (x4)|
|Meteor (x3)||Meteor (x5)|
|Triple (x3)||Triple (x5)|
|Ultima (x3)||Ultima (x5)|
Gather what spell stocks you need - it'll necessarily involve a lot of running around and searching for these unseen spell troves, but it can be worth the effort. Keep in mind that these draw points aren't as lucrative as others, as even with a Magic score of 200+ you'll probably only draw 2-5 spells from each one. They make up for this with sheer abundance.
When you've depleted the draw points or are otherwise ready to move on, continue with the next task you wish to complete. Chronologically in the guide, that means seeking out and subduing Jumbo Cactuar.