Sekiro: Shadows Die Twice - GOTY Edition

For this you have to get one of the first four skill tree's LAST skill.

Shinobi Arts -> Shadow Rush - I highly recommend this! (And later Shadowfall)
Prosthetic Arts -> Living Force
Ashina Arts -> Ashina Cross
Senpou Temple -> High Monk

You don't have to get all skills on the skill tree, just reach the last one! You can skip some, you only need to make the line to the last SKILL. The shortest is Ashina Arts skill tree with 15 skill points to the end.

There is an early game farming spot, just after the first "horse-riding" boss. You need the loaded Axe which can be found in early Hirata Estate. Without it you can not break the hat that they use as shield! They are very strong (high hit damage) and uses poison, they can kill you easily, so if you make a mistake, do not fight, run away, rest at the idol, and start over.:



PS: In fact killing them is a side quest in the game. From the idol if you go across the field - back where you came from, up on the stairs, then right and go in the building, talk to the old man, who ask to kill these rats. When you killed them, go back for reward (Ashina Esoteric Text > Ashina Arts skill tree). Later in the game you will meet him again at Great Serpent Shrine idol and if you acquired any of the above mentioned LAST skills, he will give you the Mushin Esoteric Text which opens the last skill tree, the Mushin Arts.

In this video guide I will show you how can upgrade one of your skill trees early in the game. The first area is in the Hirata Estate after getting inside the walls and getting to the idol. From there I make a path straight forward and kill and much as i can stealthily. I unlock the gate first, which has to be done by going around first. Continue to keep heading like your going to advance in the level. Kill the mini boss first before farming and then kill all the guys in the area as well. Then make your way backwards around the left path and finally the area next to the idol. Return to the idol and reset, keep grinding until you feel it is enough. The next area is the Ashina Depths, make sure you kill the this mini boss as well before farming. Just jump down to stealth kill this guy beneath you then grapple over to the guy with the cannon for another stealth kill. Then grapple over to the guys in the hut, kill the guy on the right with stealth then just attack the guy to the left. Just return to the idol and reset. Grind to your hearts content.
Edit: I thought I mentioned in the video but I will add it here as well. You need to complete one skill tree all the way to the ultimate move for the achievement.