They're beatable, but you need quite the damn good units. As of currently i am only doing the Golden Armour G map 1* (killing all necessary enemies) and for that i need...
-3 gold+ soldiers
-3 gold+ mages
-3 healers
-2 good duelists, preferably one princess
-1 AW high level Bernice or better
-Prince

And yes, all of them at max level and some of them AW'd.

So yeah, the requirements can be a bitch. I sometimes 3* the map if my Berna can assassinate 3 armours quickly that i normally skip.