I personally don't think Kylo getting a beat down is jarring, at all.
For a start, there's the Bowcaster shot, a weapon we've seen send stormtroopers flying.
Then there's the nature of the character; bad tempered and emotionally unstable. He admits to his dad that he's going through an internal conflict, being "torn apart". Killing Han, which he thought would bring closure, seemed to do the exact opposite. Couple that with the fact that he's partly responsible for failing Snoke, his master, it's fair to say his head is fucked by the time he reaches the forest.
I love that whole scene; it's so raw and emotional. He immediately lashes out a Rey, and Finn confronts the demon he was trying to run away from. He went to Starkiller base purely to rescue Rey, not confront Kylo. In that duel, he's always on the defensive, and only gets a hit in on his shoulder as Kylo's getting wreckless. Kylo responds and burns Finn's shoulder, before totally overpowering him and nearly cutting him in half.
The first half of the Rey fight is all Kylo; she's back peddling till they reach the chasm edge. Only then, once the Rey realizes her latest ally (the Force) can help her, is Kylo bested. Now there's an obvious boost Rey gets from further unlocking her potential, but by now Kylo is wounded in multiple places, and totally losing it. We saw his reaction to even mentioning "a girl". There's a history there, and an established fear. Though nothing was said, I'd speculate that he or Snoke have foreseen the threat she poses. He acknowledges her potential on the ledge, asking her to be trained by him, but is denied.
For a mentally unstable, wounded, emotional, wreckless and arrogant character to lose in a quick flurry of defiance, isn't too far fetched IMO.