In order to get the Height of Technique achievement you will need to get all skills from all five skill trees (Shinobi Arts, Ashina Arts, Prosthetic Arts, Temple Arts and Mushin Arts) and for this you will need to do more than one playthrough if you don't want to spend hours and hours farming XP, i actually did four playthroughs and i ended up having to farm for about 3~4 hours either way, so with that in mind i saved my save file just before getting all skills (because farming sucks and no one deserves this)

If i'm not wrong the save file have 30 skills points to spend and 3 skills in Mushin Arts tree to unlock, so you can just save your game and put my save file on your pc, get the achievement and go back to your save and play the game normally, a friend of mine already tested and it worked, he said he used this video (all credits to the creator) to help him out.: https://www.youtube.com/watch?v=Y0SrpJfQpZk

In order to reach your save file in sekiro you will need to go for: C:\Users\your_username\AppData\Roaming\Sekiro, you will look for a .sl2 archive in there, the name probably is S0000.sl2, once there copy YOUR save and paste it somewhere with other name like S0000save.sl2, with that done copy MY save in the same place where your save file was located and delete YOUR save from there, once this is done change its name from S0000skills.sl2 to S0000.sl2, enter the game and spend the skills points in the unlocked skills, this should pop the achievement for you, once this is done just quit the game and put YOUR save file in there, change its name for the original one "S0000.sl2" and delete MY save from there, the game should run normally from the place you ended you last gaming session.

The problem with "failed to load data" should be solved with the video link mentioned above, either way this is another guide on how to do this (again all credits to the creator) https://gaming.stackexchange.com/questions/348146/how-to-transfer-your-sekiros-save-file-to-a-new-account

Any questions you have feel free to text me or to comment in this guide, i will try to reply as soon as possible

My save file:
https://www.dropbox.com/s/lbpx6qp861prrp3/S0000skills.sl2?dl=0