They have no magical resistance, and I used the spell with a character that had Master Dark magic at 18 + Guinevere which should add 1/2, i. e. total of 27. The spell mostly worked, but there have been cases when an archer was clearly hit by it and still nothing happened. So the spell description is maybe just wrong? Or there is some other factor working, completely unknown.
(P. S.: These were tests against the blue and red archers. When testing it on the green ones (with 10 magic resistance), the results have been much worse. There was roughly 50% efficiency, but I met one archer that got hit 6 times by the spell and still lived. According to my calculation, there is only 0.024% chance that this happens, so I wonder whether there has been some obstruction, maybe the spell hitting the ground or whatnot? But I could clearly see the spell hitting the archer.)