The dungeon you describe works best when the non-heroic party is able to explore it in a non-heroic manner. Generally, this means at least the following:
- They can explore it at their own pace, fully able to stop / run away any time they are not satisfied with perceived or actual level of danger.
- They can, at least often enough, guesstimate the level of danger to be low enough to proceed.
The challenge then mostly shifts from direct confrontation with whatever level-appropriate opposition is behind the door you've just kicked in, to:
- Correct assessment of the surroundings and their inhabitants.
- Planning, and executing, a reasonably safe way of getting some of stuff that is to be gotten from this dungeon.
Namely, this includes determining if, and when, to leave the rest of the dungeon alone, unplundered, and turn to other dungeons. You generally don't expect a large challenging non-heroic dungeon to be completely cleared, at least not in one go.
I can, and probably will, include relevent examples.