Map: Eldin Caves
Recommended Element: Water
Heart Container: Impa
Heart Piece: Link
Skulltula: Defeat 1,000 enemies
This location recommends using water element. Impa will be the suggested warrior for this stage so let's just go along with it for now. Make sure to make an Empty Bottle badge for her as well from the Bazaar.
From the starting area, smash the nearby jars to get some Rupees. Head out and take control of the nearest outpost to the north. Once youve objective is updated, head to the Hyrule Captain's position, to the northwest of the map. To make it there, you have to break through the Stone Square keep in the middle. There should be a breakable wall along the path.
Note: There's a boulder near the breakable wall. Don't destroy it immediately. Kill 1,000 enemies first then destroy this boulder to reveal the Skulltula underneath.
Use your bombs to destroy it then clear the Square's defenders to take it under your control. This will allow Link and the allied forces to reach the Hyrule Captain. You're still required to rendezvous with him so before going to meet him, head south and capture the keep there.
After regrouping with the Hyrule Captain, you'll learn that you need to destroy the stone spire to proceed across the lava. Dark forces will appear and shortly after, an allied Bombchu will appear from the south. Head there quickly and escort it. The Bombchu will make its way to the West Square. Defeat the Lizalfos and the Keep Boss there to capture the Keep and allow the Bombchu to pass safely.
Once the stone spire has been destroyed, continue north and regroup with Link. After the cutscene, the Dark Forces reinforcements will appear from the south and attempt to pursue you. You can head there and engage them in battle. I would recommend doing this so you can get more experience and material drops. You'll also be able to protect your Keeps and outposts along the way. Once done, join your allies and take control of Tunnel Square.
After putting the Tunnel Square under your control, a red chest will appear. Open it to get a Sealed Weapon. Once done, the Tunnel Interior will open. Head there and fight your way through to the straightforward path. The Spirit of the Cave is located to the south end of the path. However, the Crystal Cave to the northeast will be opened so might as well take out the keep. Head south and defeat the Spirit of the Cave to open a Tunnel Gate.
Once done, keep killing enemies for the meantime and wait for the mission objective to update. Once you got the objective to find a Bombchu, head to the Central Keep and capture it. One Bombchu is there. After capturing the Central Keep, Wizzro will appear. You also need to escort the bombchus to the stone spires. Defeat all enemies surrounding the bombchus until they successfully destroy large stone spire. Once done, you can clear all remaining enemies.
Capture the store room where there are several jars containing a good amount of rupees and the area south of the Store Room has a boulder in the corner that contains a chest that has a Heart Container within (using Impa as the character). To complete the mission, you have to reach the cave exit.
After the battle, you'll be able to play as Sheik.