Back in March, I wrote about how to make DF fights tough.
Here is another couple of ways to make things tougher.
Mixed Enemies - Instead of just one kind of enemy, mix some together. An example of this came in my last session, but the players diffused it. Had they not managed to fight their enemies piecemeal, they'd have fought a combination of strong but slow stone golems (very high damage melee fighters) while fending off swarming ash spirits (moderately damaging blinding clouds of evil ash). The weakness of one (moderate damage, fairly obvious weakness vs. wind) is covered by the other (high damage, can walk through a Windstorm without difficulty). The strength of one (blinding attack) feeds into the other's attack (high-damage moderate skill flail strikes).
Set foes - literally put there as guards - should generally have some mix or have some terrain to make them viable foes. Otherwise it's too easy for a diverse group to stand off and kill them (or charge in and kill them) once the weak point is discerned. A diverse group of guardians, guards, assassins, or what have you, is a much greater threat due to the need to deal with multiple avenues of attack.
One I discussed before, but which needs some reinforcement:
Magical Support - Even if the enemies don't have allies, or pets, or coincidentally occurring inadvertent aid, and even if they're a homogenous mix (a tribe of orcs, a pack of stirges, etc.), they might have a supernaturally adept member. The foul bats from DF2 have their own special leader types. Certain demons have spellcasting versions. Orcs might have shamans, or holy men, or demon-summoning spirit-callers. Defensive spells (Missile Shield, Shield, Bless) or dispelling attacks (Dispel Magic or Purify Air vs. Fog or Stench) can keep the PCs from being the only ones to bring magic to the battlefield.