Kol was in a fight = he was battle ready = he had his defenses up. Using lightning from behind doesn't magically make it more powerful. The only cheap about it was that Kol couldn't block it with his lightsaber. Still Nihl is powerful enough to shit all over Kol any time he wants.
Also idk where you are at, so if you are not yet at issue #15 don't read.
Spoiler:
Nihl also effortlessly chokes Cade unconscious.