In the game, Shaak lost to Galen soundly even with the aid of Felucians, and got overpowered in the Force by Galen.
In the comic, Shaak got severely wounded by an omnidirectional telekinetic attack that wasn't even focused on her.
In the novel, Shaak held an edge over him in lightsaber combat throughout the duel, but if it were purely a lightsaber duel it still would've only ended in a stalemate and Galen still bested her from use of the Force.
Sure Galen is above your average Jedi Knight or average Jedi Master as a duelist, but what notable combatant isn't? Sure Galen outdueled Rahm in a close ass duel, but the most Rahm has is being a Juyo practitioner (so is Sirak).
Galen has contended with Vader in spars, great! Pre-Lehon Bane contended with Kas'im in spars and still would've gotten blitzed if Kas'im was going all-out.
Sure Galen matched Vader in a duel at the end of the game, but his duel with Shaak is closer to his duel with Rahm (whose nowhere near Vader), and since Felucia, Galen has been stated to have several power growths, a shift in Force alignment, an increase in skill, and a significant increase in baseline physicality, so Felucia Galen really isn't comparable to end game Galen as a duelist.