Birth by Sleep: Best Grinding Spot

This guide will be explained assuming you are in Standard Mode. Beginner Mode will have enemies at lower defense and will make the recommended starting level drop slightly. Proud Mode will probably be 3 levels higher than my recommended levels.

In this thread, I'll discuss the best grinding spot in the game and how best to approach it with each of the individual characters, and which setup to use. Radiant Garden: Reactor is the best place for each of the characters, and if the battle level glitch didn't affect your Terra or Ventus, you'll have a much easier time with this strategy. Aqua will have the same battle level stuck at 6 due to her Final Episode involvement. And in Final Episode, this area is not available, so you're stuck devoting about 10 hours to get her from level 40 to 99. Terra nd Ven take about 5 hours with their battle levels at 10 and 9.

Before starting with this at all, you should make sure you have four Mastered Mega Flare commands, you can pull it off with just one or two, but it takes more time to let the command reload. Mega Flare must be combined to make, a nameless enemy might drop it, but it's simply easier to just meld it with Crawling Fire and Fission Firaga. Mega Flare is an Ultimate class Fire command that is most effective at clearing a room of weaker enemies, and is most effective with this strategy.

Once you have the Mega Flares, you must make sure you have the Fire Boost, EXP Chance and Reload Boost abilities at least. EXP Chance douples your EXP intake when your health falls below 25%, and Reload Boost speeds up your command recovery rate at 25% health. Both make this go by a lot faster, and Fire Boost makes your Mega Flare that much more destructive. To make this easier, it's best to take all of your Item Boost abilities off, especially Treasure Magnet, otherwise, the items dropped by these enemies will heal you up and the process starts over again.
For Terra, you might want to make a Sacrifice command just to be on the safe side. Sacrifice can be made by melding Dark Haze and Warp, but is exclusive to Terra only. Sacrifice depletes Terra's health to critical by unleashing a powerful swing, which will make it easy to get his health back down if you accidentally pick up an HP orb.

Terra
Recommended Starting Level: 40
Terra will take more time then Ventus since his dodge maneuver is less controllable than Ven and it covers more distance. When you enter the Reactor area, run to the middle until the enemies appear, then Slide Dash to the upper right of the area, then unleash your first Mega Flare. After the flames clear, immediately unleash your next Mega Flare. If you didn't get all of the enemies in the first wave, or some of them disappeared, wait for the items to disappear and run to the center until they appear, Slide Dash away and clean up. If you take out all of the enemies, you'll receive around 4,500 EXP, which doesn't seem like much, but it adds up quick. After you've taken out all of the enemies, exit the area and come back in, then rinse and repeat until you're satisfied. By level 99, most enemies will die in a single hit, but his defense will still suck, so don't count on a maxed out Terra to be all that useful.

Ventus
Recommended Starting Level: 35
Ventus does not have Sacrifice, so to lose health effectively, it's best to enter the Reactor area, then exit back out to Aqueduct and let the Unversed fling it's windblade at you until you're down to critical. When you're done wwith that, go ahead and proceed with the grinding. Ventus can be easier to manipulate than Terra with Dodge Roll, but the same basic strategy applies, but instead of dodging to the upper right, it can be faster and less risky to heal by dodging back the way you came. Ventus may need to equip a Keyblade with higher magic than his Lost Memory key, Pixie Petal works great, but it seems to affect Mega Flare somehow, making it fly out as a sphere till it hits something, which will sometimes cause enough of a delay to let the enemies disappear. At level 99, Ventus will have a much better Strength than the lower levels, and enough magic to deal out serious harm, his defense will be slightly less than Terra's, so same assumptions apply.

Aqua
Recommended Starting Level: 33
Aqua won't have much trouble taking out the enemies due to her high magic level, but this will no doubt take a lot of time out of your day given the ridiculously low battle level of Radiant Garden in her story. Same strategy applies to Aqua as it does Ventus, but the Ketblade will most likely not matter at any level for her. She may not even need the Fire Boost ability depending on how low you're starting this out. You may need to Cartwheel twice since it doesn't cover as much distance as the other two charcters, but you probably only need one cartwheel to get far enough away. By level 99, Aqua's spells will obliterate everything in the room in a single hit, area of effect spells will be all you need. Her strength will be significantly more impressive than level 50, but her defense is the worst of the three, so make sure you don't leave yourself open at all.

That's all there is to it, hope this helps.