I've never understood why some think it's a case of Force power, when the attack target has always been based on any small imperfections/cracks in the body of the opponent.
That said, the Force is important in a way; it determines how good you can become as a melee combatant, which would determine if you can stop Krayt from molesting you. But that's about it.
It was in combat. They were dueling and Krayt touched him, killed him and revived him. He also sent images about Cade's future as a Sith, and Cade claimed that he saw things clearly now or some such thing. Krayt, in his arrogance, took this to mean that Cade was submitting to the dark side, so he let his guard down and Cade one-shotted him.