Maybe he knew that she wouldn't kill him. Maybe he was willing to die? Who knows.
What I do know is that the argument that Rey shouldn't be good at fighting with a staff/sword because she isn't a jedi is moronic. Again we see in TFA that she regularly carries around a quarterstaff. She uses it at one time to overpower 3 assailants on screen. You can tell that she's someone who's had to fight in a harsh desert environment to survive on her own for many years. There's no reason to believe she shouldn't be skilled with a quarterstaff or other simple melee weapon.
How often did Luke use his lightsaber before he held his own against Darth Vader, one of the most powerful Jedi and Sith ever? One who's lightsaber skills were said to compare to Mace Windu's, the only person to best Darth Sidious in a lightsaber battle. How could Luke a desert farm boy with minimal training handle himself so well against the most powerful figure (or second) left?