One way to 'push' PCs in a horror game is to use moving environmental features, a slowly advancing flood of hungry blood, with creatures in it's depths lashing out, works better than most monsters. Once they see it's a creature, that means the players know it has hit points, and it can die, so they duke it out anyway.
Larger than colossal creatures also can work, if the PCs are exposed to parts of them at a time. A monster the size of a dungeon complex can be treated as per Dungeonscape traps, where they basically can beatdown this local chunk of it, but another will be coming round in a minute, and the walls are ALSO part of it, and shift to attempt to trap and digest them.