There is a total of 42 skills (125 levels and 2,909,961 XP) across 5 Esoteric Texts (trees) that can be acquired by spending Skill Points. For this achievement, you are
not required to obtain skills sold by a merchant or awarded after defeating a certain enemy. I highly recommend you unlock every other trophy first before starting to grind XP due to the fact that bosses will give you a great amount of experience on NG+.
The best place to farm XP is Ashina Castle and the best moment to do it is after you've defeated the final boss. Depending on the ending you've chosen, the method will vary slightly.
Not Shura:*** Spoiler - click to reveal ***Travel to Upper Tower - Antechamber (Ashina Castle), go behind the folding screen and perform a Backstab Deathblow on the Interior Ministry Agent (purple ninja). Rest at the idol and repeat.
Shura:*** Spoiler - click to reveal ***Travel to Upper Tower - Antechamber (Ashina Castle), go behind the folding screen and perform a Backstab Deathblow on the Fencer (blue ninja), then go behind the Interior Ministry Agent (purple ninja) nearby and execute him as well. Rest at the idol and repeat.
The location of every Esoteric Text, their skills and their cost has been listed below.
Shinobi Arts (34 Skill Points)Location:
Skills:*** Spoiler - click to reveal ***• A Shinobi's Karma: Body (2 Skill Points).
• A Shinobi's Karma: Mind (3 Skill Points).
• Breath of Life: Light (5 Skill Points).
• Mid-air Combat Arts (3 Skill Points).
• Mid-air Deflection (1 Skill Point).
• Mikiri Counter (2 Skill Point).
• Run and Slide (1 Skill Point).
• Shadowrush (6 Skill Points).
• Shinobi Eyes (3 Skill Points).
• Suppress Presence (2 Skill Points).
• Suppress Sound (3 Skill Points).
• Vault Over (2 Skill Points).
• Whirlwind Slash (1 Skill Point).
Prosthetic Arts (31 Skill Points)Location:
Skills:*** Spoiler - click to reveal ***• Chasing Slice (1 Skill Point).
• Emma's Medicine: Aroma (5 Skill Points).
• Emma's Medicine: Potency (4 Skill Points).
• Fang and Blade (2 Skill Points).
• Grappling Hook Attack (1 Skill Point).
• Living Force (4 Skill Points).
• Mid-air Prosthetic Tool (3 Skill Points).
• Nightjar Slash (2 Skill Points).
• Nightjar Slash Reversal (3 Skill Points).
• Projected Force (1 Skill Points).
• Sculptor's Karma: Blood (2 Skill Points).
• Sculptor's Karma: Scars (3 Skill Points).
Ashina Arts (18 Skill Points)Location:
Skills:*** Spoiler - click to reveal ***• Ascending Carp (2 Skill Points).
• Ashina Cross (5 Skill Points).
• Breath of Nature: Light (2 Skill Points).
• Descending Carp (1 Skill Point).
• Flowing Water (3 Skill Points).
• Ichimonji (2 Skill Points).
• Ichimonji: Double (3 Skill Points).
Temple Arts (22 Skill Points)Location:*** Spoiler - click to reveal ***From Main Hall (Senpou Temple, Mt. Kongo) idol leave through the door on your left and enter the cave with a white tent. At the very end of this cave, there's a three-story pagoda. The Esoteric Text is inside.
Skills:*** Spoiler - click to reveal ***• Devotion (3 Skill Points).
• High Monk (4 Skill Points).
• Most Virtuous Deed (4 Skill Points).
• Praying Strikes (2 Skill Points).
• Praying Strikes - Exorcism (3 Skill Points).
• Senpou Leaping Kicks (3 Skill Points).
• Virtuous Deed (3 Skill Points).
Mushin Arts (20 Skill Points)Location:*** Spoiler - click to reveal ***Given by Tengu at the Great Serpent Shrine after learning the final skill of any Esoteric Tree (after defeating Genichiro, but before defeating the Divine Dragon). Alternatively, given by Isshin after eavesdropping on Isshin and Emma. If the Divine Dragon has already been defeated and the Esoteric Text has not been retrieved, it can be obtained from Emma in Kuro's Room.
Skills:
Thanks to nramsey34 for pointing out a few mistakes.