Here's a theory for you. In The Unifying Force, Jacen tapped into the raw Force. What if Galen managed to tap into the raw Force? Galen is strong in the Force - strong enough to defeat those Force users as chilled monkey pointed out. But he forgot to add Vader who other than Sidious was the most powerful Sith. If Galen tapped into the raw power as Jacen did in the The Unifying Force, Galen could possibly defeat any Jedi and Sith alike. Note the word possibly.
Galen was trained by Vader, but Vader underestimated his Force potential when he "unleashed" him. Galen was like Mara when she was the Emperors Hand - living a life of violence. Where Mara was a true assasin, Galen at the time was Starkiller - a wrecking ball of the Force who really abused it. He had complete disregard for life until after Vader betrayed him.