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.