By and large the main way they're a threat is either by immediately rushing and ganking anyone that leaves the fortress, or against my actual military, then they simply use the Extreme Russian method of throwing so many units at one person that that person passes out from over-exertion and can be killed by the 20th opponent of the fight
I swear to god if these bats had their population tracked like any of the surface civilizations I'd have wiped all of them out ten times over (although what really annoys me is that the World Map says I'm at peace with the bastards)
I think the most I've had was around 150-200? Guy was a hunter, so of course he had a lot just from that, but also defending against giant crows and the odd siege here or there.
I've seen colossi with thousands of unique kills before but rarely more than a few hundred on my guys, even with the way the subterranean invaders are.
My spearmaster in a fortress past had about four hundred miscellaneous subterranean victims, and almost eighty named kills. Most of those were subterranean critters who earned themselves a name, but many of them were also dwarves from my own fortress