[Taqwus]

If somebody does substantial research and site searching, they *should* get something which isn't terribly bothered by light infantry that could have been purchased from the get-go. That's the whole point.
That said, between Curse, the large number of affliction-causing items, and the availability of AP power (Weapons of Sharpness doesn't even cost a gem, for instance), the finite equipment slots, the existence of specific counters like the unresistable Wither Bones, and pure ugliness like Gifts from Heaven or even Gate Cleaver, nothing's invulnerable.
Even a Tartarian with an outrageously good kit isn't a meaningful threat if somebody's just lopped off both its arms with Picas and Procas, and somebody else hit it a couple of times with Vision's Foe.