I just wanna suggest you Ancient one we found interesting with herald/guardian :
Ancient one - Yibb-Tstll (more clue tokens found on map, harder to fight)
Guardian : Hypnos (he make more clue tokens appear on map each turn)
Herald : Ghroth (add 2 doom tokens on start of game) or Lurker - cause if someone take pact this Ancient one will have crazy amount of Hits
This way you will have only 9 turns to seal all gates and numbers of clues appear is at least 2. If you wanna spice the difficulty you can draw only FOCUS 1 investigators which will make your team focused on win by closing/sealing gates other than fight with this Ancient one.
? Cherrypick your choice of investigators to boot.
Yet another rule misinterpreted. Sigh... :::smacks face:::