Even after all this time, Devourer Below remains one of the most difficult scenarios in Arkham Horror.
Due to the encounter deck recursion and the large number of cards that add doom (roughly half the deck), you're going to at best get 8-9 turns before Umordoth spawns. The other half of the deck that isn't doom cards are enemies, four of which are very hardy.
This would be enough to make this challenging, but the random location debuffs make it dangerous to even explore.
All in all, this scenario should honestly be skipped when introducing people to the game unless they like getting stomped on. Most, even good viable parties, will end in failure on this scenario.