I guess I'd go hardline. The big balancing factor in the RAW for Mist Form is that the presence of any allergen snaps the Mist Form-ee back into their normal form.
HMHVV is a world-known threat, and many of the HMHVV species are both very strong and dangerous, it stands to reason that the big-boys have methods tailored to take them down, some of which would trickle down, and are quite cheap.
I'd just declare that sufficiently advanced sunlamps create sunlight, and all that entails. Fighting a group of intruders, and one of them (who appears to be casting high force spells, a mage) puffs into smoke, turn on your sunlamp flashlight/flood light! Especially if there is news going around of similar occurrences, it makes sense that most prepared enemies would be prepared with sunlamps. That would make use of Mist Form at least require premeditation and planning to create appropriate blackouts or take out key features of an emplacement, probably with help from the other team. Instead of dropping into the fight directly with stunbolts and quick death, it might be advantageous to start blasting lights with physical spells.