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.