I've posted this as a reply to a topic on BGG: for those who feel that the Ancient Ones from the base game are not challenging enough, and don't want to use/don't have the epic battle variant from the Kingsport expansion, here are a couple of suggestions to make the game more challenging.
1. Remove all skill checks from the Ancient One's attacks in final battle. The investigators' losses become unconditioned.
2. Always strive for a sealing/closing victory during the game.
3. For each sealed gate you're short of to score a sealing victory, the Ancient One's combat rating is increased by (-1).
So if you're playing against Yig, and there are only three sealed gates on the board when he awakens, his combat rating improves to (-6).
?