Head to the end of the Undead Settlement
The boss is located near the end of the Undead Settlement. Head there and you’ll go through a cut scene.
It’s an optional boss
Curse of the Rotted Greatwood is actually an optional boss which you can skip. However, the fight is fairly easy and very rewarding.
Kill the undead first
There will be a spawn of undead surrounding the boss which you’d need to kill immediately before you head to the boss.
You’ll need 3 Estus Flasks
On a basic level, you’ll need 3 Estus flasks, which would aid you if you get attacked. However, this is a fight you can win without receiving any damage provided you have a long-range weapon. (See How to Beat Dancer of the Boreal Valley in Dark Souls III)
Attack the sacks on his body
The only way this boss takes damage is through attacking the sacks on his body and arms. Causing enough damage will lead the boss to stomp the ground, which would start the second phase of the fight.
Attack the tree’s right hand first
You should start the fight, after killing the undead redeyed enemies, by dashing to attack his right hand where the sacks are. (See Dark Souls 2: Scholar of the First Sin Boss Fights Guide)
Attack once the knee is up
Once the tree lifts up its knee, charge in and attack the sacks on its body. This will trigger the second phase.
Dodge a lot in phase 2
During the second phase, you’ll be forced to dodge a lot because he will throw himself on the ground after standing upright. Once he falls on the ground, attack the sacks around his body.
It’s harder to hit the boss in phase 2
It’s harder to hit the boss in phase two because he will continue to repeat his attacks by standing and hitting the ground. He will also squirt acid on the ground around him, which would be wise to dodge.
Keep an eye on the sacks
Keep an eye on the sacks when you’re dodging and take every opportunity to attack him.
The boss drops 7000 on a new game, Transposing Klin and Soul of the Rotted Greatwood.
How to Defeat the Curse-rotted Greatwood – Dark Souls 3
Dark Souls 3 Boss Guide: How to Defeat the Curse-Rotted | How do you beat the Curse-Rotted Greatwood?