Curse-rotted Greatwood achievement in DS3

The Greatwood Tree is the third boss you will fight and he is very easy to kill. Greatwood can be heavily damaged by hitting the eggs on his body starting with the front for the first phase of this fight. Blocking does not work in this fight so rolling is your best bet to survive this.

Fight the mob of enemies as they approach you or get the Greatwood to kill the enemies.

Once you have done Enough damage to the eggs you will fall through the floor with the Greatwood (you won't take damage so don't be scared that the drop will kill you when you see it..). Now in this phase it's just you and him! Attack his back, side & front eggs remaining and at about 15-20% he will fall over giving you a little time to attack him freely (did that for me atleast).

Below is a video from powerpyx.