If you're viewing a map, Monster Island is the island at the top center. You can get here as soon as you have the whale. You may wish to wait awhile, though...
Monsters: Gonghead, A. Sludge, Chorking, K. Sludge
Gonghead can cast both Freeze and Cure 4, and has 1000 HP. You'll have to keep whacking away at it until it runs out out of both AP (this will take a couple of rounds) and HP. Can drop Extract.
A. Sludge can cast 8.0, and has 900 HP. Death always works on it, and Drain and Hush usually work on it. If you don't have Death, Hush it. Can drop Extract and Van. Extract.
Chorking has 25 HP, has a defense rating of something like 500 points (you'll do one point with each attack), and is immune to all elemental spells. Use an item to cast spells, or cast Chop-Chop. It can use Poisen Breath and Rot Breath, so you have to kill it quickly. Can drop a Charm or a Shiny Bracelet (very rare). That Shiny Bracelet is a real good reason to go hunting the suckers (take Sten's Efreeti form).
K. Sludges appear in pairs or trios. They have 1000 HP. They can cast Death. They are faster than you are. If you don't have G. Dragon, they'll probably wipe your whole party. If you run into these with G. Dragon, use it and at least one spell that damages groups. If you don't have G. Dragon, your options are: 1) Take your best shots and hope for the best, 2) try to run, 3) if Sten's an Efreeti and lives long enough, he can "Sweh" them away, or 4), die. K. Sludges are worth more experience than anything else in the game, and can drop a Sacred Shield or Army Glove. This is a very rare occurence.
You can earn huge amounts of experience here. The chest in the fishing spot contains a Love Bracelet.
You can get here as soon as you can fly. It's the island east of Evrai. Visit the building there -- it's the best armor store in the game -- but don't go monster hunting until you have G. Dragon AND have earned a good deal of experience, probably at Monster Island.
The monsters here are Dark Bringer, Cyclops, and Night Rider.
Dark Bringer -- you fought these at High Fort. Slaughter them, or use "Sweh" to try to get a more worthy foe.
Cyclops has 950 HP, and is very vulnerable to magic. Death always works. Even without Death, this guy is easy.
Night Riders come alone or in pairs, and have 1000 HP. They can cast Typhoon, Hail, and Fireball, and can critical hit for a huge amount of damage. With luck, power, and a good stratagem (like using Holy Shields to cast Shield spells, using Attack Up spells, and using Cure 3), you can triumph over a lone one without resorting to G. Dragon. If your facing a pair, use G. Dragon and some group attack spells, or die. Night Riders are worth more experience than anything on Monster Island (not counting the K. Sludge). They can also drop a Medusa Shield or a No Twin Rapier, though you'll have to win many battles to get either.
If you use a dragon attack at any point while you're on one of these islands, retreat back to a resting place as soon as the fight ends. Or use White Fruit or Minnows to bring up Ryu's AP to full. Your call.