@MeesaBack (who himself learned of this elsewhere) shared this *very* easy way to fully deflect Isshin's One Mind without having to rely on the umbrella.
All you have to do is sprint towards/into Isshin after the opening slash, then keep sprinting forward as you deflect the 4 airslashes (which you can do by pretty much spamming). After this if you keep sprinting Isshin's finishing strike will miss, but you can stop sprinting to deflect it instead.
Sekiro: Shadows Die Twice Statistics For Alpha545 Gaming
At present, Alpha545 Gaming has 10,391 views spread across 71 videos for Sekiro: Shadows Die Twice, and about 7 hours worth of Sekiro: Shadows Die Twice videos were uploaded to his channel. This makes up 58.35% of the content that Alpha545 Gaming has uploaded to YouTube.