Wow, me and two my friends just defeated Cthulhu. It was our second game against him(first was with the larger group and we lost it).
The way we won it was dramatic. He was 1 friggin doom token from awakening and then I started sitting at the Asylum and spamming Arcane Insight at every upkeep, restoring lost sanity during the encounter, while others tried to do something with the opening gates. There always was two Witch House mythos cards which will result in the awakening of Chtulhu(dead end, we had absolutely no means to fight him, it was an autoloss), and third card which triggered the monster surge or was blocked by an elder sign. I prolonged the game length as much as I could, when suddenly mythos card resolved which required investigators on the streets to test fight, lose 1 stamina on fail and gain 1 exhibit item on 2+ successes. And he got this type of exhibit item which removed doom token from the doom track and placed it as the elder sign, which is technically saved us, because the 3 top cards of the mythos dack eventually became 3 witch houses. At the second note, one of them was a Blessing of Nodens environment, which dropped the seal cost by 2 clues. Silas had 3 and Yorick had 1 clue and 3 monster trophies. After the gate at the Witch House opened, we had a 2 free turns due to monster surges not adding the doom tokens to the doom track, sealing Cthulhu away with 12 doom tokens on his doom track.