While I can see where your coming from Ren got hit by Chewy's gun which we saw numerous times throughout the movie destroy people. And Finn got a few hits on him as well. So he wasnt at full strength. Also we can assume from what we heard throughout the movie that he has not fully completed his training. Both are extremely powerful characters. And we have to remember Vader is the standard for strength as far as force users go. So its not really fair to compare to Luke vs Vader to Ren vs Rey.