The Curse-Rotted Greatwood is a tricky boss to fight because there are only certain parts of him that you can hit in order to deal damage. When the fight starts, you can run straight to the boss in the corner of the arena to trigger him. This will allow you to bypass the other enemies, and you can also bait the Greatwood into swiping them to death.
You’ll soon notice an egg sack jiggling around the Curse-Rotted Greatwood’s crotch. That’s your target. I highly recommend two-handing your weapon as taking hits with your shield will most likely deplete your entire stamina bar and break your guard at this point. During the first phase, you’ll want to hit the egg sack as much as possible, but be sure to leave plenty of stamina for rolling away from the boss. Whenever the Greatwood lifts its arm, it’ll swipe the area right in front of the egg sack, so make sure to roll away once you see the windup.
Another attack you need to watch out for is the Greatwood’s body turn. It will turn slowly, but if you’re anywhere near its limbs, those can hit you for massive amounts of damage as well. These swipes are best dealt with by rolling away. Lastly, when the Greatwood lifts its body up, you’ll want to roll away as well; the butt slam will damage you if you’re anywhere near him, and he’ll also spew a yellow liquid that drains your health slowly if you’re standing in it. Once you destroy the egg sack, the Greatwood will break the ground, and you’ll continue the fight in the basement.
During this second phase, you won’t have to worry about the adds anymore, but the Curse-Rotted Greatwood does get a lot more dangerous as well. He will have egg sacks on his crotch, his back, his leg, and his arm. You’ll need to destroy all of them to win the fight. At this point in time, a large white hand will also burst out from his belly. It’s worth noting that this hand can be attacked for extra damage too.
Be careful when you’re attacking the sack on his back; the Greatwood will occasionally try to fall on his back to crush you, and this attack cannot be blocked. However, it is also a good opportunity for you to get some hits on the sacks on his arm and leg. You’ll also need to watch out for the large hand when you’re attacking his crotch. If you’re at mid-range from the boss, the hand can grab you and slam you on the ground, taking away a large portion of your health. The hand is significantly less dangerous if you’re close to the boss, but remember to watch out for the liquid-spilling attack as well.
If you have a bow, this can be useful for hitting the sacks on his back and his crotch, but the sacks on the arm and leg will have to be attacked manually. My best advice is to take this fight as slow as possible. The Curse-Rotted Greatwood itself is pretty slow, so it’s easy to hit him, but if you get caught by any of his attacks, he can end the fight very quickly.