Yeah, I steer clear of darkling shamans, disciples of Pariden, haunters, dark mages, etc... Nowadays I obsessively check everyone who may be a caster for despair, coal stones, wellspring, and the like...
On "easy," and within ten squares of a starting city, I'm figuring this is a bug.
A single unit of archers can usually counter them well. I always make sure to bring one.
I've got a sovereign using a bow. But the thing had a fair amount of hit points, high initiative, and both a warrior and archer with it. And was listed as "weak." I think in trying to make the early game a bit harder Stardock got a wee bit too enthusiastic. There needs to be a progression of difficulty, sucking in the player with a strong chance of success (and some strategy required) in the early game. Turn them off with battles everywhere that are just too difficult, and they may decide FE's another instance of a game keyed for really sophisticated strategy players, and not for themselves.