Galen claims later on in the novel that Vader held back during their early sessions, so it wasn't all-out. Galen had speed, knowledge of Vader's form, and Force abuse to aid him in defeating Vader.
In raw skill, Galen is hardly very impressive. He has some hype from the databank but it's not reliable if you look at his track record.
Originally posted by Jmanghan
Funny, cause in the game, Marek, almost killed him, and while there are doubts for Sidious, you can't say Marek didn't almost kill him, he sabered his helmet off and force-pushed him through a window.
Well according to Sam Witwer, the plans for TFU3 were for Vader to show his true power and completely batter Starkiller.
Originally posted by SunRazer
That was never stated in the novel, and it's pretty illogical that it wasn't, but there was seemingly a big deal out of Galen knowing Vader's form.
I don't see why it would need to be stated, Vader literally taught Galen to duel. Obviously he knows his techniques and style inside and out. Meanwhile, Vader held his own skills and true ability back from Marek.