The safe word is "combined arms". This is extra-true for fantasy elements in a modern setting.
Dragons and tengu are helping with recon for their squads along with setting up mortars and sniper nests in hard-to-reach places. The Foundation is also constantly switching between costly but hard-hitting mechanized troops (tanks, fighter jets, etc...) and smaller, nimbler organics.
Similarly, I wanted to explain the presence of quarterstaffs, swords, and clubs as complementary to modern-age firearms and artillery. The only unique advantages, however, are their ammo (practically infinite), relative quietness, ease of manufacture, and tolerance to damage.
But when to use them?
They're still fighting at long distances and you probably don't want to risk getting closer. So, if I'm already involving genetically-engineered creatures, such as dragons, I might as well add something that's specifically designed to screw over firearms but can be more effectively dispatched with weapons like clubs, quarterstaffs, swords, spears, etc...
I don't have any idea what such a creature would be, not even a clue.
They can be terminated with firearms, but it's less effective than using the aforementioned melee weapons.
The aforementioned melee weapons can damage the creatures badly enough to incapacitate or terminate them.
The creatures can attack and have an actual combat role in the enemies' ranks besides justifying medieval weapons.
Knowing Worldbuilding, someone would probably have said that the answer is children. You don't want to waste your ammo and the quarterstaff gets the job done just as well. Well, now that kind of answer is invalid |)
So, how would this creature work?