Freeing Korok Forest

Overview
Freeing Korok Forest is the first scenario that occurs in Chapter 3. It can be unlocked after completing "The Yiga Clan Attacks!" scenario.

The scenario begins with the Divine Beast Vah Medoh appearing, where Revali will have to destroy 27 Outposts that are in and over Lake Mekar. The game will be over if Vah Medoh loses all of its HP to the monsters.

Once Revali destroys all the outposts, Link and his friends go through the Lost Woods. They were getting lost, when they find Hestu, a Korok.

Hestu guides them through the Lost Woods to Korok Forest in exchange for the characters protecting him. They then find out that there is a Malice swamp blocking their path. They then must defeat the Hollows of Mipha, Daruk, Revali, and Urbosa in order to destroy the Malice swamp. They find the Master Sword, but then find that Astor, the Prophet of Doom, is in the way. Then, the objective changes and Link must defeat Astor in order to win. Note that Link is the only playable character here, and during the Astor boss fight no one else can attack Astor.

Being Guided by Hestu
When protecting Hestu, order one of the characters(order by pressing +, than pressing the A button) to stay with Hestu, while making the character you are controlling to patrol ahead. This way, the characters can make sure there is no danger ahead of Hestu, while Hestu can easily be protected by switching to the character guarding Hestu.

The Hollows
Once the characters see the Malice swamp, Hestu should become playable. Try to use Hestu as much as possible, as he has a higher level than the other characters.

Another thing to keep in mind is to collect as many rods and apples as possible. They will become particularly useful in the Astor boss fight.

Astor Boss Fight
Astor will be completely different from all the other bosses, and will be extremely powerful. Even if Link has the Master Sword at his disposal, Astor will still be hard to defeat. My recommendation is that Link should only fight Astor when he has his Weak Point Gauge out. If Link tries to attack Astor while he is attacking, Link will lose many hearts.

Try to make the Weak Point Gauge appear by either just fighting him after Astor does an attack that makes his Weak Point Gauge appear, or use one of the rods in order to stun Astor. Using stasis against Astor when his Weak Point Gauge is out will be particularly helpful.