Night trolls are always a good bet for better humanoid enemies, though they get much weaker when you hit the higher levels. Intelligent undead humanoids are another good bet (both natural or player-made, though the latter will have no equipment at its creation) due to their boosted toughness and special abilities. If you don't mind investing a fair chunk of time and effort, you might try powerlevelling and gearing up a humanoid adventurer or three, retiring them, then seeking out and fighting them (bonus points for making them vampires, intelligent undead, necromancers, werebeasts, or some combination of the above).
Failing all else (and if you don't mind using it), DFHack's assign-skills script can be used to make whatever humanoid bandits or soldiers you find into stronger fighters.
Vaults are also a good bet for high-level humanoid fights, assuming your world has them. You may want to steer clear of the Archangel, though.