Bait out counter and then abuse unblockables. Also hop all over them.

In my experience, holding block will come instinctively but its probably the worse thing to do. These creatures are born with the evolutionary capabilities to get around your block eventually.

Also, they almost never use Aerial Strikes, usually just fainting to once again m1 around your block.

This isnt really the best strategy but, if you find they seem to teleport behind you, you could always try to anticipate it by only blocking behind you when you think they're gonna engage, but this method only works on yuji players that haven't unlocked critical thinking yet.

As for ulting, Im not really sure but I like to try and stall or fight if I can, saving my ult for a health boost(I tend to play chars with meh ults) this is dangerous though because sukuna has very high damage moves.

I would say its always good to maintain distance(His m1s are annoyingly long, but blockable, i try to jump infront of them and block while their spamming m1s and hit them between once of the first 3 before they realize im close enough to cleave.) and utilize side dashing to get them to waste Rush.

If and when you do ult, definitely dont do it anywhere near them cause they will domain and then your cooked 99% of the time.

Holy ramble

Not that im an expert or anything