This is cultist base master level on nightmare difficulty with the following console command:
ai_haste_overriderate 2
I don't know exactly what it does, but as you can tell the gameplay is a little bit different :) (I guess since it's not too obvious, it doubles demon movement speed and attack speed/rate, no idea if it does anything else, and it definitely doesn't affect the slayer and it doesn't affect projectile speed, but there's a command for that as well).
I've played with this setting before, originially I was planning to do the entire campaign with it about 1-2 months after the game released, but it was too exhausting so I gave up on it. I only did a couple of attempts and my only attempt that got past like the first level died at the end of ARC Complex to the marauder after I missed the ice bomb glitch. At 6:15 you can see the only marauder fight in this master level, it's a lot easier when you have space to move around, unlike the end of ARC which is quite brutal in a straight up fight. The hardest part of the campaign itself is actually the gladiator, and marauders in general are difficult after the ice bomb fix.
I've also played this master level with this setting before, but that was a few months ago now, usually I died a few times then, especially in the pit. This was actually my first attempt at recording a deathless playthrough, so that's fortunate.
This might be unexpected, but the most dangerous demon on this setting might just be the hell knight. An absolute terror because of it's ability to cover the distance quickly and annihilate you immediately, and is therefore always the first priority to kill.
Thanks for watching.