Frost Stones are a synthesis ingredient only found in Kingdom Hearts 1.5, not in the original. And these are only dropped by the rare Grand Ghost found in Monstro’s Stomach.
This heartless has a 100% chance of dropping a Frost Stone, but it can only be defeated by using healing items in your menu.
I recommend using just Megalixirs, since you can actually synthesize them yourself, and these should kill the Grand Ghost in one shot. This also increases your chances of getting multiple Frost Stones, because using a Megalixir in this fight gives you a 100% chance to get two stones with the chance of a third.
That said, you can also farm the Grand Ghost by using regular Elixirs since those also guarantee a Frost Stone drop.
In KH1.5 you can use Frost Stones to synthesize the Moogle Badge, Crystal Crown, and Seven Elements Shield.
Frost Stones can only be dropped by the Grand Ghost heartless in Monstro’s Stomach, located right where you fought the Parasite Cage the second time.
Don’t go into this fight swinging, though. Because he can’t be damaged by physical or magical attacks.
Instead you have to go into your item menu and use a healing item to deal damage.
And you’ll want to use these items fast, as he’ll run away after a while in the fight.
Your total damage and Stone drop chance all depends on which specific healing item you use.
Defeating him outright with items like Mega-Potions or an Elixir guarantees that he’ll drop a Frost Stone.
But if you want to farm more per battle, you’ll have to use better items.
The absolute best item to use is a Megalixir.
It’ll deal 300 damage and this always gives you at least x2 Frost Stones as a drop, with a 20% chance to get a third stone as well.
But if Mega-Potions or Elixirs are easier to get then you can definitely use those for effective farming too.
There are three items in the game that need Frost Stones for synthesis.
Moogle Badge: An accessory that gives you +6 HP, +1 MP, +2 AP, and enhances your magic and summon damage.
You can forge it with List II and x1 Blazing Stone, x1 Frost Stone, x1 Lightning Stone, x5 Mythril, and x3 Orichalcum.
Crystal Crown: Another great accessory which offers +6 HP, +1 MP, +3 AP, and enhanced magic and summon power. Make it with List IV and x5 Lucid Crystals, x1 Power Crystal, x1 Shiny Crystal, x3 Blazing Stones, and x3 Frost Stones.
The recipe for this item in the original KH1 game is much different and doesn’t require Frost Stones.
Seven Elements: A strong yet odd-looking shield for Goofy, exclusive to the Final Mix/Remix version of the game. It focuses solely on magic and raises Goofy’s strength by +4 and MP by +3.
You can synthesize this shield with List VI and x3 Blazing Stones, x3 Frost Stones, x3 Lightning Stones, and x5 Dazzling Stones.
If you’ve got spare Megalixirs lying around (or if you’re willing to synthesize more) then these are worth farming.
And you should try your best to get enough Frost Stones to craft all the recipes you can with them.
Especially since these are one of the easier items to farm when you know what you’re doing. So getting lots of stones shouldn’t take too long, and you can even bulk synthesize a bunch of Megalixirs to help with the process.
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