Sekiro: Shadows Die Twice - GOTY Edition

To get this you need to have at least beaten the Folding Screen Monkeys boss in order to unlock the Puppeteer Ninjitsu ability. From here travel to the Senpou Temple, Mt. Kongo sculptors idol. Exit the building and head north east down the cliff, go left past lone monk on the broken bridge and drop down, you should see a lone rat enemy near a mechanism. Stealth kill him then press to activate the Puppeteer Ninjitsu ability (you need to have this equipped), he will then reel in the kite.

Progress through the area and then through the next building. Upon exiting you should see a very large tree in front of you, you need to grapple to the top of this. Once at the top you will see the kite used by the rat, you can grapple this and traverse the canyon. Once across jump off the ledge to the right and use the grapple multiple times until you reach a cave, this contains the Sunken Valley Cavern sculptors idol.

Travel through the cave until you reach the Sunken Valley Passage, you will see what looks like a plank overhanging the cliff. Walk to the end of this and you should see the Great Serpent below you, simply jump on and perform the deathblows when prompted and the serpent is no more. Enjoy your achievement!

Any issues please let me know!

*EDIT* You need to have encountered the Great Serpent in both the Ashina Castle Outskirts as well as the Gun Fort before he'll appear at his killable location.

In this video guide I will show you how to defeat the Great Serpent. Now the first things you will be grabbing is the gun fort key which is in Kuro's room after defeating Genichiro. Next you will want to go defeat the Folding Screen Monkeys to get the puppeteer ninjutsu. After that head back to the senpou temple, mt. kongo spawn. Once you spawn head out the door then to the right, if you look down you will see a monk by a broken bridge. Now look even further down and you will see a one of those rat guys next to a kite in the sky. Use the puppeteer ninjutsu on him so he will activate the kite. Now leave and find the great serpent shrine that Kuro mentions when you talk to him. It is around the other side of the castle of where you came from. From there progress to the gun for and kill the centipede mini boss. Once you unlock the door you will see a bride and most likely try to cross it and get attacked by the snake and die. Now fast travel back to senpou temple, mt. kongo and follow the normal path until you climb back up into the building with the undying monk and go out the window. You will now see a big tree in front of you, climb it and use the kite to grapple over. You will follow the path around the cliff until you meet badger again. Just follow the path down and you will see a wooden plank to walk out on. Walk out and look down to see the snake, now target the snake with RS and once you jump you will see the execution symbol. Press it to do an execution and you will be prompt to hit it again and get the achievement.