Well I guess Ash is a soldier, so she'd know how to fight. Also, she's not flippen mad. Jack's not as trained in combat as Ash, but her rage is the thing that really gives her an edge . . . that and her biotics. Stop your complaining people. We don't know if the fight is over yet.
YOU DIDN'T SAVE SHEPARD ASH! JACK DID! And the only reason you can hit her right now is 'cuz she's destabilized. And Ash, you're soooo gonna loose 'cuz... well... Jack's destabilized! RUUUUUUN LIKE HEEEEELLLL! Abandon ship! She's gonna blow!
Jack's still recovering from being hit by the Reaper beam plus she pretty much emotionally collapsed from seeing Shepard injured and near death. On top of that, Ashley comes over to open up old wounds. Yes, I would say she has reason to be destabilized. ^^