Dark Souls 3 has a great deal of bosses, and Greatwood is one on those which you can just skip. However, it is highly advised to defeat him as he drops the Transposition Kiln which you need to perform soul transposition. With it, you will have access to some of the best equipment in the game.
Now if you do go in for the kill, there are a few things that you should keep in mind. The only way to damage it is by attacking the sacks you see on its body and that it will not be easy to lock on to the weak spots with ranged attacks like sorceries. Also, note that you can trick him using Alluring Skulls but you don’t have to in order to defeat him.
While on the first phase of the battle, you will notice that there will be some other enemies in the arena. Although they are potentially fairly easy to kill, you should try to avoid them as much as you can and kill them only when they interfere with your battle with Greatwood. That is because not only they will keep respawning but the boss will also do a pretty good job at killing them, helping you clear the way.
1. Going full melee
Fighting this boss in melee is probably the best and easiest strategy for most players. So you will want to make the best out of it.
In order to do so, once the boss activates, charge to his left hand and start attacking the egg sacks there. Once you destroy it, get back and anticipate a kick from one of its legs. Wait for it to raise its knee and charge at the sacks located on its groin while avoiding the kick. Be ready for a counterattack with his arm and either roll away or block it. Keep attacking as described above and soon enough it will trigger the second phase of the battle.
Once Greatwood breaks the floor and you both drop down a pit, you will no longer have to worry about additional enemies as they will cease to respawn. Now that it’s just you and it you will have to be patient. As long as you keep your distance you will be safe and able to determine the location from the rest of the sacks.
Notice the ones on his back, groin and his right leg and keep an eye out for the white hand. Whenever you feel like an attack is coming from it, avoid it and you will have enough time to land a couple of strikes on your target.
If you want to play it safe then you can wait for Greatwood to stand up. It will telegraph his Stand-and-fall attack which is easy to dodge. Charge at it from its left and aim for the white hand. Since you will only have a 5-second window of opportunity, you can use Pine Resin with lightning or fire to increase your damage. It may take you more time to defeat him like that, but it is the safest approach.
2. Limit the use of magic
Although Greatwood appears to be a tree of some sorts it has no vulnerability to fire. What makes magic and other ranged attacks a tricky approach is the fact that the boss is vulnerable only if hit at the sacks or the white hand. Locking on will not work as it will always lock on the torso instead of any weak spot.
As a final note, try to keep your health full and don’t rush it. Follow the above and it will be easy enough.