In Sekiro Shadows Die Twice you can find Prosthetic Tools that can be used as special weapons with your prosthetic arm. But you may wonder how do you upgrade the prosthetic tools?
It’s quite simple:
Step 1: Get the Mechanical Barrel from defeating the first main boss “Gyoubu Masataka Oniwa”. He’s found in Ashina Outskirts, Ashina Castle Gate Fortress idol. The best way to defeat him is to block his attacks until he does a heavy strike that leaves him defenseless for 2-3 seconds. That’s enough time to hit him twice. Repeat this process. It takes a while but is a safe strategy. You can also grapple on to him by pressing L2 / LT when the green circle shows up on the boss. This also stuns him for 2 seconds and lets you land two hits. So essentially, you could just let him ride around while you block and wait for a chance to grapple, then repeat this tactic.
Defeating him drops the Mechanical Barrel which unlocks Prosthetic Tool Upgrades.
Step 2: Return to the Sculptor at the “Dilapidated Temple” and he will unlock the upgrades for you. They require some money and the materials “Black Gunpowder” and “Scrap Iron“. Later on you need other higher-end materials such as Scrap Magnetite, Adamantite Scrap, Yellow Gunpowder and others. These can be found in the world at fixed positions but can also randomly drop from enemies.
Fully upgrading all Prosthetic Tools to their limit unlocks the “Master of the Prosthetic” trophy or achievement.