Originally posted by MythLordYep. Not only was he unready for the attack, he was in the process of trying to talk her into coming over to the light.
She is. She really is.And she one-shotted an unready Luke, who was hindered by the nexus, so...
" I've met your kind before, Luke said. Listen to me, Gethzerion turn away from the darkside before its too late."
"She pointed a finger at luke, and before luke even recognized her evil intent, a ripple of force slammed into him."
Also at that point in the story Luke was naive, always doubting himself, and unsure about everything he did.
"Luke felt unsure of his power. The force. Where did it really come from? Yoda had said that life created it, that it was energy. But could luke use it in good conscience? If he was drawing energy from other living things, sucking them as if he were some leech bleeding them dry, how could he really justify what he did."
Also Luke acknowledge that Vader had been holding back in their previous fights and could have done the same to luke if he wanted.
"And there was the further matter, in his battles with darth vader and the emperor, luke felt he had never truly tested his powers to the limits. Vader saught only to turn him, had kept luke alive. Yet luke had no illusions that Gethzerion would be so lienient."
"So this is how it would have been had Vader tried to kill me."
To emphasise luke being naive at that point. He acknowledge that gethzerion wouldn't be as lenient as vader and the freaking emperor, and still goes to face her unprepared, defenseless, and tries to talk her into coming to the light.
So to sum it up, Gethzerion on a nexus powering her, one shotted a young, inexperienced, naive Luke, who was unsure of himself and powers and went into the fight unprepared and defenseless.