My Nyarlathotep rules :') use them if you dare.
At the beginning of the game, do not put the masks in the cup. Random masks spawn when new gates open instead of the normal monster, after they're defeated the first time, they go into the monster cup for random selection afterwards. Masks do not effect the monster limit. They can be temporarily removed by gate closings or feds raid arkham type cards. This variant is particularly fun if you have all the box expansions (and have the tougher masks from Dunwich and Kingsport). The Beast, The Skinless One, and The God of the Bloody Tongue make town navigation quite a challenge, along with the lesser masks that will at the very least jam you up in the streets ::laughter:: and once you get a gate surge or two, the town gets really flooded. In the meanwhile, the guy who destroys clue tokens (not his actual name) gives you a little minigame to play as you try to kill him. Add to this challenge :'D throw in Tulzscha as the herald, and then you'll also have elusive cultists destroying elder signs while being defended by the masks, while the elusive mask is destroying clues. Yeah, you might still be able to stockpile enough clues and items to take Nyarl down, but maybe not. Not if the endless(ly annoying) cultists and masks slow you down or take you out. Anyways, it should still be possible to beat Nyarl in direct combat, but I think it will be considerably more difficult.
Also, change Nyarl's attack effect to a loss of two or three clue tokens (depending on how easy you think it is). I would give you an exact number, but I've never tried this with the Herald before (I'll try it Saturday night probably— while going for a combat victory).