Although the Curse-Rotted Greatwood is an optional boss in Dark Souls 3, the Ashen One needs to defeat it to acquire the Transposing Kiln, which makes it a high priority on the hit-list of all players interested in boss weapons.
Therefore, knowing how to take this boss out quickly and efficiently is something that will be helpful for every playthrough. There are many ways that the Ashen One can destroy this early-game Greatwood fiend, even with starting equipment and only a few upgrades. Cutting down the Curse-Rotted Greatwood is appropriately similar to felling a tree in the forest, though when this one falls, all hear it.
Melee Strategy - Use A Slash Weapon
Early Slash Damage Weapons
Slash damage is 1 of the Greatwood's 3 main weaknesses, which means that it will possibly fall to one of the many curved swords or katanas in the game. Some that can be acquired very early in a playthrough are the Scimitar and the Uchigatana. The former can be bought from the Shrine Handmaid for merely 600 souls while the latter is dropped by the Sword Master near the entrance to Firelink Shrine after he is defeated.
How To Clear Phase 1
The battle with the boss is divided into 2 phases, the first of which is not very challenging as long as one keeps away from the hollow farmers that crowd the courtyard. Simply burst 3 of the Curse-Rotted Greatwood's disgusting egg-sacks to initiate phase 2, where the stone floor gives way and both the player as well as the Greatwood fall into the dank pit used by the Mound-Maker covenant.
How To Defeat The Boss In Phase 2
Once in the pit, the Greatwood will be a bit more dangerous, as it will now have a dextrous new arm reaching forth from its torso-bark that was cracked by the fall. Try to take out the remaining egg-sacks on the Greatwood's trunk and limbs while keeping an eye out for when the monster stands up to do its big slam attack or when it releases its caustic acid.
After the egg-sacks are gone, the huge tree-demon will still have some HP left, meaning that it is time to take on the pale and slimy hand in the front. It can be wily; both tough to hit and possesses annoying swiping attacks, though even mediocre rolling or blocking can allow one to avoid damage. If the light and heavy attacks with one's weapon do not reach the arm very well, try jumping attacks for the extra height.
Melee & Ranged Strategy - Use A Thrusting Weapon
Melee Thrusting Weapons
Another of the Greatwood's 3 weaknesses is thrust damage, which, like slash damage, is pretty common among many weapons. Early melee weapons, such as the Longsword that players who chose the Knight class begin the game with (and similar straight swords), often have a powerful thrusting maneuver that can be unleashed with the heavy attack. Using such a weapon requires essentially the same tactics as with curved swords or katanas: avoid the big slams, swipes, and acid while popping the gross egg-sacks and hitting the pale new arm.
Ranged Thrusting Weapons
Those desiring a less up close and personal fight with the Greatwood will be glad to know that bows deal thrust damage with their arrows, making either any of them or the early game crossbows, easy tools to help dispatch this boss in any run. The manual aim can be especially helpful in taking out the egg-sacks from a mostly-safe distance and also hurt the white arm from out of its swiping range.
Spellcaster Strategy - Use Fire Spells
Pyromancer Builds
Pyromancer builds will be able to make kindling out of the Curse-Rotted Greatwood with their fire spells. Out of every boss in the game, the Greatwood has the lowest fire damage absorption at -110%, making them exceptionally easy to kill with flames. Even the most basic pyromancies will make short work out of this boss, just be sure to keep a safe distance and avoid AoE attacks while casting. Keep in mind that the Curse-Rotted Greatwood is immune to all status ailments, therefore pyromancers should leave Poison Mist and Toxic Mist out of this battle.
Sorcery & Miracle Builds
Even spellcasters who are not dedicated to pyromancies can make use of the lower-tier spells, such as Fireball, which lobs a small orb of flames, or Flame Surge, which emits a mid-range plume of fire. Neither of these requires more than 6 Intelligence or Faith, making any sorcerer or miracle build capable of using them.
Cheese Strategy - Use Pestilent Mist
Sorcerers with 30 Intelligence or more will also have access to everyone's favorite boss-cheesing spell: Pestilent Mist. It can be bought from Orbeck of Vinheim after he is sent to Firelink Shrine following the player encountering him on the Road of Sacrifices; he'll begin selling this spell after he is given any Sorcery Scroll.
This cloud of vapor depletes the health of any who stand shrouded in it by a percentage of their max HP, including the player, so be careful not to wander into this white mist while evading the Greatwood's attacks. The fog does not actually hit anything, meaning that it will not rupture the egg-sacks, allowing the player to defeat the wicked tree without triggering the more difficult phase 2.