[chaos_alfa]

Originally Posted by Jehuty

It sort of was in the last episode on the previous arc. I think it depends on where you're struck for it to count as a one-hit kill. Think of it as critical hit areas.

The reason why Kirito could defeat Heathcliff with one hit was left out of the anime.

Spoiler for Comparsion to the Light Novel:

Asuna screamed tearfully from behind me. But I didn't look back. I shifted my right foot backwards, brought my left sword forward while lowering my right sword, and entered my stance.

Kayaba manipulated the window with his left hand and readjusted our HP bars to the same level. It was just before the red zone, where a single, strong hit would decide the fight.

After that, the system message [changed into mortal object] appeared above his head. Kayaba then closed the windows, pulled out the sword he had dug into the ground, and raised it behind his cross-shaped shield.