i think you forgot that Rey has been handling and using her staff for god knows how long while Luke on the other hand had no training and still managed to kill Vader. Luke was able to do it because the force taught him to and Rey's knowledge is a combination of actual experience fighting with a weapon as well as the force
Luke also did a lot of off-screen practice between the films.
Yes, the force does help guide users in fighting with a lightsaber (particularly in deflecting blaster shots), but the user also needs practice and usually training.
After all, there are several different 'forms' or styles of fighting that need training. Mace Windu was one of few to invent and codify a new difficult form. Force Users couldn't just learn an entire form from the force.