Lumiya fought Luke at the height of his power to a stand still. That's Talzin and not Gethzerion. We never see anything like that from her. Granted it seems Lumiya has not faced a Nightsister before and lightning would be a problem, but she trained under Vader and Palpatine. You think they never mentioned Force Lightning to her? It's not a new concept. Her cybernetics also enhance her speed and strength. Her ligthwhip gives her the ability for ranged attacks. She uses it like a lightsabre, and would probably just get out of the way of the lightning. Why is Gethzerion's prep so much more than Lumiya's? Lumiya was a master at espionage, traps, bombs, and poisons. She can manipulate your perception with Force phantasms. I think she can think of something for a Dathomiri witch.