She was beating Finn, who again, has lots of feats in the RT I linked. Her armor is also far better than Rex's, seeing how Rose's blaster shot merely bounced off her shoulder. We don't really have solid 1v1 feats for Rex in TCW but in Rebels he was losing to Pryce in a H2H battle.
Rex definitely won't demolish Phasma, if anything, Phasma demolishes him.
Just one thing, Rose's blaster...is that even described as what type of blaster? It looked more like a Holdout, which those aren't particularly powerful compared to actual blasters such as rifles or pistols.
If it was a shoulder shot, then I don't see how that would indicate she would take it anywhere else and not be phased, IE: A direct shot to the body, a shoulder shot isn't nearly as damaging and the way the armor is it could deflect. In fact, it even says with her armor being able to stop weak blaster shots.