4.1 Freeing the Second Monkey
On your way to “the Temple deep within the forest”, you will find that the gate to the tunnel entrance is locked. Talk to Coro and he will gladly give you the Small Key.
Coro will ask if you would like to purchase a Bottle of Oil for 100 Rupees. This would be a good idea, not just because you will gain a new bottle for future use, but the next part of the mission will automatically take all of your oil.
When you get to the Poison fog a Monkey will steal your Lantern and clear a safe path to the other side. Stay close and defend the Monkey from enemies. When you re-obtain your Lantern it will be empty.
After the Hero’s Shade teaches you the Ending Blow, continue along to path to reach the entrance to the Forest Temple. Burn the web with your Lantern to enter.
If you don’t have any spare oil you can buy or steal some from Trill’s shop.
The first room is pretty straightforward, free the Monkey and follow her through the door.
To get to the northern door in the second room, light the four torches on the central platform. Before exiting through the door, there is a Treasure Chest with the Dungeon Map just east (west in the Wii version / Hard Mode) of the door.
After the cutscene, return to the second room. The Monkey will prompt you to jump and swing across to the east (west in the Wii version / Hard Mode) door.
Kill the Bomblings to the north of the entrance to blow open a hidden area. There are some Pots, one with Oocoo in it.
Follow the Monkey until you reach the north door. Go through the door, continue north across the bridge, and enter the next door. There will a Treasure Chest with a Small Key in it, directly west (east in the Wii version / Hard Mode) of you.
Run back to the room where you left the Monkey and follow her to the locked door.
Roll into the totem pole three times to knock the caged Monkey down. Dispatch the Bokoblins and climb the ramp back towards the door. The Monkeys will help you cross the broken bridge.
4.2 The Third and Fourth Monkeys
Return to the second room; the Monkeys will now help you get to the opposite door.
Climb the ramp and throw a Bombling into the Deku Like’s mouth to get to the northwestern (northeastern in the Wii version / Hard Mode) door. Use another Bombling to destroy the boulder blocking the door.
Roll into the northern totem pole to knock down a Treasure Chest containing a Small Key. To get past the Tile Worms, run across the south tiles until you reach the other side of the water. Light the north and south torches. Approach the south torch first, all tiles touching it are safe. The north torch is guarded from the east (west in the Wii version / Hard Mode).
After freeing the third Monkey, return to the previous room. Kill the Skullwalltulas on the vines to the south. Jump to and climb the vines to reach the next door.
Kill the head of the Big Baba, leaving a Deku Like boy. Then throw a Bombling into the Deku Like’s mouth to kill it and retrieve the Small key.
After exiting the room with the fourth Monkey, use a bombling to kill the Deku Like just to the east (west in the Wii version / Hard Mode). Drop down to where it was to find a treasure Chest with a Piece of a Heart.
Now that you’ve freed the fourth Monkey, return to the second room once more and exit through the north door. Swing across and enter the door to the north.
4.3 Ook the Baboon
When Ook throws his boomerang, roll into the totem pole that he is standing on to knock him down. While he is stunned, strike his butt. Repeat this process until he is defeated.
4.4 Grabbing the Big Key
Use the Gale Boomerang to spin the windmill above the door three times to unlock it. Exit the room and use the Gale Boomerang to manipulate the position of the bridges to the west (east in the Wii version / Hard Mode). Run across the the bridges and use the Gale Boomerang to cut down and free the fifth Monkey,
Return to the second room and use the Gale boomerang to knock down the Treasure Chest in the middle of the room. Inside is the Compass.
Swing to and open the east (west in the Wii version / Hard Mode) door. Climb across the vines along the southern wall to get to the Boss Treasure Chest behind wooden bars. Looking south at the pillars, use the Gale Boomerang to lock onto the windmills on top of each pillar in the order drawn on the ground. Inside the Boss Treasure Chest is the Big Key
4.5 The Last Monkeys
Gamecube/ Normal Mode: Northwest, northeast, southwest, southeast.
Wii / Hard Mode: Northeast, northwest, southeast, southwest.
Jump across to the North door and exit to the outside. Use the Gale Boomerang to re-position the bridges and enter the next room.
Rotate the bridge to get onto it, then rotate it again to gain access to the west (east in the Wii version / Hard Mode) room. In here, use your Lantern to burn the web over the north hole in the ground and drop through it. Free the sixth Monkey and return to the previous room.
Rotate the bridge again and go through the north door. Rotate the bridges and clear the Skullwalltulas as you climb to the west (east in the Wii version / Hard Mode) side of the room to enter the next room.
Use the Gale Boomerang to grab a Bombling and throw it into the Deku Like’s mouth. Jump across to the south and open the Treasure Chest to get a Small Key. Climb to the far side of the room and grab another Bombling. Time your toss so that it will blow up in the air next to the rocks on the upper level. Climb up and use the same trick to get a Bombling (stand close to the edge) and destroy the rock to free the seventh Monkey.
Return to the room with the rotating bridge in the middle and go through the east (west in the Wii version / Hard Mode) door.
With the Gale Boomerang you can clear the area of Tile Worms and safely run through the tunnel to the northeast (northwest in the Wii version / Hard Mode). Climb up the vines and use the Gale Boomerang on the windmills to free the eighth Monkey.
Return to the room where all the Monkeys are gathered and swing across to the Boss door.
4.6 Twilit Parasite Diababa
Use the Gale Boomerang to guide Bomblings into Diababa’s mouths. Once the main body is revealed, Ook will swing back and forth holding Bomblings. Guide the Bomblings into the middle head to knock it down and reveal his eye, strike the eye while it is exposed. Repeat until Diababa is dead.