Walkthrough:Kingdom Hearts/Darkheart3/Atlantica

Atlantica

 * Items Obtainable: Mega-Potion x3, Mega-Ether x4, Elixir x3, Cottage x3, Mythril Shard x5, Mythril x3, Orichalcum x1, Torn Page x1, Crystal Trident, Ansem's Report 3
 * Magic Obtainable: Thundara
 * Keyblades Obtainable: Crabclaw
 * Abilities: Mermaid Kick(shared)
 * Enemies Encountered: Search Ghost, Screwdriver, Aquatank, Sea Neon, Sheltering Zone
 * Bosses Encountered: Shark, Flotsam, Jestam, Ursula

Atlantica Overview
You'll encounter a new enemy type here - the Sea Neon(30 HP, 22 ATK, 19 DEF, 8 EXP), which can be killed easily with Fira. They drop enough MP balls to make up for the cost of casting on them. Clear out the enemies here, then open the clamshell by hitting it and use the Save Point. You won't be able to use the single Trinity in this world for a while, so keep Ariel in your party for now. In the room beyond, search for treasure in the clams. Check for a red clam on a red - defeat the enemies around it, then cast Fira on it to obtain a Mythril Shard. Head towards the wall to find a blue shell - hit it to obtain a Mega-Ether. Move towards the red clam again and check towards the geyser, then to the right you'll see a water spout and a blue shell. Hit the blue shell to receive a Cottage, then look towards the geyser to spot another shell. Hit this one to obtain an Elixir, then move towards the red clam again and swim down to the base of the pillar it's on. There's a shell here that contains a Mega-Potion - grab it, then check the hole in the nearby wall to find another shell with Mythril inside. Follow the tridents on the walls into a trench where you enter the Calm Depths - head up to the edge of the current and look downstream. Enter the cave on the right to enter the next new area, and defeat the enemies here. Follow the Tridents until you reach the Undersea Cave. Enter the hole near the bottom of this area to find a shell holding a Cottage - swim up to enter the Undersea Gorge. Take a left as you enter, and drop into the trench to find a dark blue clam - hit it with Blizzara to obtain a Mythril Shard. Turn around and stay low to find another shell with a Mega-Ether inside. Follow the tridents until you see one pointing to a tunnel. Check to the left and enter that tunnel first, hit the clam and grab the Mega-Potion, then backtrack and follow the trident. You'll encounter the Screwdriver(48 HP, 22 ATK, 19 DEF, 15 EXP), which are the same sort of cannon fodder Soldiers are on land. Clear the room and continue forward. Look down and to the left to spot a shell containing a Cottage. On the right from the entrance is a shell with a Mega-Ether in it. Turn and look for a yellow clam in the distance - hit it with Thunder to get a Mythril Shard. Turn and face the conch in the middle, swim to the right of it until you pass over a walkway, then drop down to find a shell holding an Elixir for you. Head to Triton's Throne. Watch the cutscene, then turn around and re-enter the Throne area. Hit the clam to reveal a Save Point, and use it to save your game. Return to the Undersea Gorge and ride the dolphin for a bit and dismount at Ariel's Grotto. check the boulder blocking the Grotto to instigate a fight against the Sheltering Zone(90 HP, 22 ATK, 19 DEF, 20 EXP), which are giant Sea Neons. When you beat them, they split into a swarm of Sea Neons, unless you kill them with a Fira spell - I'd suggest that, as it also nets you the munny and MP balls. Inside the grotto, sink to the bottom to find a chest with a Mega-Potion. Hit the barrel opposite of it to get some MP balls and possibly an Ether. Head up and check the chests to find a Cottage and a Torn Page. Exit the grotto when you've got all the goods and head to the Undersea Cave. Head through the hole at the bottom of the Undersea Cave to reach the Calm Depths - head upward through the current into the Undersea Valley. You could also hit the urchin on the wall with Fira to create a shortcut. Find the dolphin here and ride him to a new area - the Sunken Ship. Before you enter the ship proper, swim down to find another ship and through the hole in the hull to snag an Elixir from the chest there. Head to your left after you exit to find another chest that hold a Mythril Shard - now head into the ship. Move forward some, and make for the chest - the Shark comes up and breaks through the window. The chest contains a Crystal Trident - when you get it, turn and look in the corner by the stairs to find a chest with a Mythril Shard. Hit the debris under the stairs to find an opening, and a clam that holds a Mythril.

BOSS FIGHT: Glut

 * Stats:200 HP, 22 ATK, 19 DEF, 100 EXP
 * May drop Hi-Potion, Ether or Mega-Potion when defeated

This is an optional battle. When you leave the ship, you will notice t he shark patrolling the area. Cast Aero to begin with and start attacking, staying to his side to avoid being bitten. If you can get him to charge at you and hit a wall, it will stun him. After you defeat Glut, you may receive Hi-Potion, Ether or Mega-Potion.

Atlantica Overview II
After you fight Glut, head to the left of the boulder with the weird rune on it, and swim to the left, entering the hole that leads to the Undersea Gorge. Hit the geyser with the keyblade to release a chest containing a Orichalcum. Head back to Ariel's Grotto, and check the indention on the wall to place the Crystal Trident. Watch the cutscene, and head back to the Palace area, fighting your way to Triton's Throne. Be sure to save when you arrive. You'll face off against the Aquatank(120 HP, 22 ATK, 19 DEF, 30 EXP), which hold Screwdrivers on their bellies. Kill them off to make a ton of munny. Head back to the Sunken Ship and enter through the hole on the right of the entrance to the grotto. If the Shark returns, cast Aero and pummel him. Head to the rock with the rune on it, and check the thing behind the ship next to the rock - Sebastian helps you here. Enter the Den of Tides and stay to the right as you head through the fork to enter a room with a Save Point and Mega-Ether. Try to be around level 35 before attempting this next area - when you're ready, backtrack and take the other path of the fork to find Ursula.

BOSS FIGHT: Ursula, Flotsam, and Jestam

 * Stats:Ursula: 450 HP, 22 ATK, 19 DEF, 1000 EXP
 * Drops Mermaid Kick when defeated
 * Flotsam/Jetsam: 60 HP, 22 ATK, 19 DEF, 150 EXP

Hit Ursula's cauldron with spells until it backfires and hits her, stunning her so that you may take the opportunity to deal her a good deal of damage; when running low on MP, use Flotsam and Jetsam to restore it. After you defeat Ursula, you'll receive Mermaid Kick.

Atlantica Overview III
Exit the area and head back in to find a sea urchin near the clam that contains a Mythril - hit the urchin with Fira and wait to get it. Head to the Cavern Nook and use the Save Point, then set Aero to a shortcut key and from the Sunken Ship, head to the Calm Depths. Use Mermaid Kick to head across the hall against the current to the hole labeled ??? and enter it.

BOSS FIGHT: Ursula

 * Stats:900 HP, 22 ATK, 19 DEF, 1500 EXP
 * Drops Ansem's Report 3, gives Thundara when defeated

This is one of the toughest bosses in the game. Begin by casting Aero on Sora. When Ursula opens her mouth very wide, use Mermaid Kick immediately, swimming away from her as she is about to suck you in. Also, she shoots bubbles at you which are easier to avoid the farther away you are and she uses a Thunder spell against you, recognized by her saying 'time to teach you some respect.' The easiest way to defeat her is to get stuck behind her head, which is difficult considering that she faces you almost constantly. The best time to attempt this is right after she shoots the energy beam from her mouth, as she will be tired and will not follow your movements. Once behind her, hit her as many times as possible before you become unstuck. After you defeat Ursula, you'll receive Thundara and Ansem's Report 3.

Atlantica Overview IV
Now that you have Thundara and Ansem's Report 3, head back to Ariel's Grotto and Sora will seal the keyhole and obtain the Crabclaw keyblade. Head to a Save Point and jump back to Traverse Town and head to Merlin's Study to use your Torn Page.

Hundred Acre Wood IV

 * Items Obtainable: Power Up, Defense Up, AP Up x2, Mythril Shard x2, Mythril x3, Orichalcum, Dispel-G, Thunder-G, Rare Nut x5

Enter Pooh's house and talk to him. Light the fire with Fira, and you'll receive a Mythril. Head outside and enter the new area that opened east of Pooh's House. Talk to Tigger and Roo, then follow them to the stump. Tigger wants you to jump from stump to stump, just like he does. Use the same stumps in the same order to reach him. Do this three times to get to the next mini-game, Tigger's Giant Pot.

Tigger's Giant Pot
Talk to Tigger again, then talk to Roo. You need to hit the nuts at the giant pot to break it open. Use High Jump and hit the nut at the peak of your jump to increase your score. You'll need to break the pot in under 30 seconds to obtain the special ability later. There's also a lot of items to collect before heading to the next area of the game - take a look in the tree trunk where the pot was to find an AP Up. There's a log on the ground, hit the middle of it to receive a Mythril. Talk to Tigger again, and he'll get on the seesaw. Ask him to bounce with you, and you'll land on a branch - jump up the branches on the right of where you land and look for the tree with a hole in it. Land on the canopy surrounding the tree and look left to find a rest chest - it holds a Mythril Shard. Jump to the tree with a hole in it, then jump through the hole - this knocks out a Dispel-G. Head back and use Roo on the seesaw to reach a branch with a chest holding a Thunder-G. Jump to the branch below where you land to obtain it. Use Roo again to return to the branch, and take the Rare Nut to give to Owl for a Power Up. Use Roo again, and take the other Rare Nut to Owl for a Defense Up. Check the tree stump in back of the area to the right of Pooh - step on it to make the geyser near the seesaw rise up. Use the geyser to get another Rare Nut, which you can trade in for a Mythril Shard. Use Tigger on the seesaw, and jump to the branch on your right to spot the next Rare Nut - give it to Owl for an AP Up. Head back towards the log and use the stump here to raise a piece of bark on a geyser. Use High Jump to get to it, and grab the last Rare Nut from the canopy nearby. Give it to Owl for an Orichalcum. When you begin to leave, the Torn Page will transform into a Mythril. Now, head out to Halloween Town. It's the Level 6 Battle World.