To add to Prototype B's solution, which did a wonderful job of telling you exactly how to defeat her, I found a way to make this fight about as easy as they come in this game.
While fighting Stacey, you may have noticed that there is an orange juice and a vodka in the container entrance to where you fight her. You may have also noticed that there's a save point directly behind that. See where I'm going here? Pick up both of those, then go save your game. Load up your save file and they will be there again. Pick them up, save again, rinse and repeat.
And if that doesn't help enough to make her an easy boss to defeat, you can also go save your game at ANY point while fighting her and when you return, she will still be at the same health level!
With these little tips here, you're going to find her to be one of the easiest (if not the easiest) psycho in the game.
Hope this helps!:)