Galen was likely going to destroy Palpie in their force lock. Unfortunately his love for Juno and his friend clouded his heart and mind and he ultimately lowered his defenses and caused an enormous explosion to allow them to escape.
Except he is. Even flagged from having fought through Imperial forces on the Death Star, Marek was capable of blasting Sidious and engaging in a force lock with him.
Don't see how given Galen has actively demonstrated superior lightning to Valkorian and actively defended himself from superior lightning ( Sidious's ).
But it's mostly because I know exactly what arguments he'll use, he'll ignore quotes stating Galen was no match for Sidious or that Vader is on par with/beyond the more powerful starkiller clone, (which he'll also ignore), he'll pretend that Sidious was giving it his all when Galen "ragdolled" him, pretend that Galen's lightning is an actual threat to Vitiate in combat, (whereas someone like Vitiate has actually shown he can defeat strike teams and Vader-level force users with his lightning,) etc.
__________________ ”You presume limits to my power. There are none.”
"Lightsabers clashed inside of the Emperor's observation dome, but Starkiller was ultimately no match for the power of Darth Sidious. Bombarded by Force lightning, Starkiller did not fight back but instead unleashed all the power of the Force within him, causing a tremendous blast that shattered the Emperor's tower and caused enough of a distraction to allow Eclipse and the Rebel Senators to escape."
Unless Marek plans on an heroing during this battle, Vitiate will absolutely destroy him.
__________________ "There is only Revan. Only he can shape this galaxy as it is meant to be shaped."
Of course Galen would not be able to match Sidious when he has to worry about allowing his friends and loved ones to escape not to mentioning exhausting his reserves fighting through hordes of Imperials on his way through the Death Star.
I mean the text even states that he actively lowers his defenses to allow his friend's to escape.
"They raised their blasters to gun down the Rebels as they fled up the Rogue Shadow's ramp. "No!" the apprentice cried, dropping his defenses to strike one last time at the Imperials."