There are ways to do it, but if the DM really wants to do it in a way that makes sense, it is necessary to start at least by the time the PCs are 4th level or even earlier.
One is to have a number of artifical conflict generators out there ranging from power groups, religions to activities. The last can include a number of arts and (blood)sports arenas ranging from the small time competition to inter-planar ones with all the logistic (like gathering competitors, gambling, etc.) and potential events that come with it. One should not overlook religions providing adventures by givng directions to handle anything from stopping planar invasions to dealing with conspiracies to rescuring souls from various nightmarish planes.
Second is to have a number of natural conflict generators out there ranging from mundane weather, supernatural weather and zones, to planar breaches and gateways. This can include such things as the Far Realms with its reality bending and aberration spewing effects.
Third, have the opponents and groups operate in mixed types and classes and have them act with coordination. If it works for adventurers, it will work for monsters as long as they can be made to work with each other.
Fourth, you may want to look at various game supplements for monsters (and their tactics). If you want to have opponents who have some degree of optimizing of the classes, take a look at the "Powergamer" books from Goodman Games (I think) (actually that would make a lot of players unhappy as they give pretty good advice and simple tables).
Fifth, have missions/adventurers that force the PCs to go without rest, retreat, re-supply. Sort of the variant of being "trapped in a haunted house" except perhaps in Raveloft or Hell equivalent.
Sixth, have opponents that are occasionally ruthless to the point of having "bomb" type effects. Having low-level minions with single use Disjunction effect devices, "suicide belts" (for the fanatics who really do know they get a reward in the afterlife), etc. PCs will be very upset against these kinds of opponents, though it could be argued that they are not unimaginable to various groups and beings out to crush adventurers.