Immortal kylo ren in tier 6 rey jedi training.

It's part of the event, you need to have Finn die. Take his mods off, only use basics. Once he dies off, Rey will come out of stun if she was stunned, then once you get Kylo low enough you will get another ability to grab a lightsaber. The fight finishes once you grab the lightsaber, you don't kill him.