Ever heard the term hyperbole? It is more likely then not Jaina used that when describing how Luke fought, since he did have a hard time with the unimpressive Welk. Sadow ripped out the cores of stars with the same technology he kept with him at all times. Strongest Jedi doesn't meant strongest force user. Ragnos kept all of those people who could throuw stars under his belt for over a hundred years. To say Luke is the strongest is unfounded, although he would give anyone a hard time.
Point is that Luke was exausted by the time he fought Shimrra, meaning Shimrra isn't almost equal to Luke, making him a good bit weaker then the likes of Sadow and Ragnos. I'm not saying the Sith wouuldn't lose,it just won't be overkill on the ground.