Rebonack, first mounted and then as a Blue Iron Knuckle.
Reward
The Raft opens Eastern Hyrule; the third Crystal grants a free level-up.
Optional
The palace contains point bags and red-potion statues; take them only when the route is safe.
Missable
No unique pickup is missable before the palace seals, provided you take the Raft before placing the Crystal.
Numbered 100% route
From North Palace, travel southeast toward Rauru, smash the boulder, and take the right fork into the graveyard.
Step onto the isolated center tombstone to identify King's Tomb, then walk straight south from it to fall into the hidden tunnel.
Move right in the tunnel and cast Fairy to reach the high ledge. Cross the island's desert tiles and enter Island Palace.
Descend and move through the eastern block corridors. Use Downward Thrust against Doomknockers and Armored Stalfos, and collect the Keys hidden under blocks and on the exterior ledge.
At the lava room, take the center Key and clear or bypass the Bots before they knock you in. Defeat the Blue Iron Knuckle guarding the Raft and claim it.
Backtrack west past the entry elevator, use a wall statue for a red potion if needed, and continue through the Wolf Head corridors.
Downward Thrust through the block floor for the boss-route Key, then descend the western elevator. Use Jump to reach the optional point bag before the final combat rooms.
Defeat Rebonack, take the Key, place the third Crystal, and use the Raft at Mido's harbor to sail east.
Boss strategy
Downward Thrust onto the passing horse while Rebonack charges.
After the mount breaks, treat the rider as a Blue Iron Knuckle: bait high and low guards, then jump-slash the opening.
Shield reduces the punishment for a mistimed approach.