I would argue that "underdogs who succeed through superior tactics" is in many ways the
default for
Dungeon Fantasy. Typically, it's assumed the PCs will use
force concentration (don't split the party!) to overcome superior forces through
defeat in detail (an adventure involves a series of separate combat encounters rather than one big fight). If you read professionally published adventures, they tend to be careful to justify this approach, for example using unintelligent monsters, monsters supernaturally bound to specific rooms, monsters who hate each other, and of course the good old "the Big Bad is overconfident". One way to up the challenge is to make the PCs work for it—making them use hit-and-run tactics, force them to finish fights quickly before reinforcements arrive, and so on. Though if the thing you want is for PCs to avoid combat, the thing I'd do is choose monsters with great combat stats but mediocre IQ, Will, and Per.