Why is Rey so strong?

Well the issue is she's quick, so it's not so easy as just "lol, just hit her first", although that is the proper strategy. At high levels she can attack before she is killed, which grants her the Foresight buff....and once she gets that she becomes much more difficult to kill, especially if she gets to hide behind someone like Poe.

I find that if I can take her out first round it's easy, but if she manages to survive odds are I will lose. You really need a high speed team to combat her, and not all of us have access to GS, Rey, Dooku, and QGJ simultaneously. Personally I have GS and QGJ and am working on Dooku now.