https://www.pcgamer.com/sekiro-mod-makes-the-game-easier-and-unlocks-the-frame-rate/

Sekiro isn't the worst of FromSoftware's PC ports, but its graphics and display options are few and its limits many. As always, modders pick up the slack. This mod from uberhalit unlocks the frame rate, adds custom resolutions with 21:9 widescreen support, lets you tweak the FoV and lots more. Oh yeah, and it also lets you change world and character speed, making the game considerably easier.

If a boss is giving you a hard time—inevitable, really—you can use the mod to slow down the global speed by your desired number but keep the player speed the same, making you lightning fast in comparison. It's a cheat rather than the addition of difficulty modes that some players have been asking for, but at least it might help you get past that brick wall you keep smashing your head against.