My first post here so thinking i might share a little tip, since i haven't seen any post similar.

I struggle with the big boss fight (Irish's Greatest Fear) at the end of HOS, played on death march with upscaled on as well. I found the best way is to counter this guy by Aard sign (plus quen sign here and there), the Aard disables his charging and high speed advantage pretty well. Basically here how it goes:

Process: keep a distance from him, provoke him to charge you and Aard his ass which cancels his charging ability, then hit him up 3 times >> back off >> get to the back >> keeps hitting him again around 1,2 times then roll away creaing distance >> repeat.

Pros: its a safer way (duh), with smaller margin of errors compaing to counter him in sword fights (which you can mess up pretty easily)

Its not difficult to time the Aard sign inbetween his charge, you don;t have to stock up too many potions, just stamina regeneration is fine for me! shorter learning curve than other methods imo.

Cons: highly repetitive, required hyper focus on his charge and timing. you can't really change between the Quen sign and Aard sign too often because again, it might take your focus away and this guy can one shot you, i'm always at Awe how fast he can be!

Let me know if it works for you or you have better methods!, have fun!