A veteran Sekiro player, GroeneWolf420, has done the unthinkable and defeated Isshin: The Sword Saint by using abilities instead of "walking, running, jumping, or mikiri countering," according to a Reddit post. To be clear, this means facing one of FromSoft's most fearsome bosses and somehow parrying through everything, even thrust attacks that are not meant to be parried in the conventional fashion (via GamesRadar).
You can watch the entire fight, which is just under five minutes in length, and honestly, it looks rough. Using attacks and abilities to close ground on Isshin and deflect attacks with precision timing instead of relying on mikiri counter for thrusts, which can deal a ton of posture damage to your opponent. There were a few times where GroeneWolf420 was on the back foot during the fight, but on their 45th try, they somehow managed to pull it off and defeat Isshin.
But it looks like this fight was just the start. In the thread below, GroeneWolf420 asks, "Are there any other bosses you guys want me to fight like this?" Someone suggests the next opponent should be Inner Isshin with all the restrictions of their first fight. Although, it looks like we'll have to wait a bit longer to see the outcome of this fight: "I'm still doing it," GroeneWolf420 says. "[I] had to beat the game yesterday again to activate charmless [but] I'm still doing it right now."
Another player suggests that the Great Shinobi Owl would be a worthy adversary: "I am gonna say fight Owl, but with thrusts. I know there are openings that Owl has where you can thrust, so it would be educational for me."
Some other onlookers chipped in to try and give more helpful tips and tricks for future fights. A couple of people recommend using iframes (invincibility frames) to their advantage. "At the start of phase 2, when Isshin takes the spear from the ground if you iframe the moment he stomps the floor, you can dodge that," a player says. While jumping isn't allowed during the fight, grappling or dashing will afford you some breathing room.
Sword Saint without walking, running, jumping or miriki countering from r/Sekiro
Despite Sekiro releasing just over five years ago, there's no DLC or story expansion in sight. Which is probably one of the reasons some seasoned players think up brutal and impressive ways to take on the game's bosses. GroeneWolf420 even admits: "I really need a DLC."
Right now, it feels like Elden Ring's DLC Shadow of the Erdtree is the closest thing Sekiro players will get to any expansion. While that may seem odd, Hidetaka Miyazaki actually said that "Sekiro was a big turning point" for FromSoft games, and many subsequent games take inspiration from its fighting systems. Shadow of the Erdtree's bosses fight much the same as Sekiro's in that they are more aggressive and less forgiving. Shadow of the Erdtree even has a Sekiro-like leveling system, just so players can't steamroll the DLC.
