Vader's former apprentice had been mid-charge when Mara threw the wave at him. He didn't have time to summon his own wave to counter it, so he did the next best thing. Gathering his strength, he leapt up into the air and grabbed onto one of the few metal catwalks above the floor. He didn't stop there though. Deactivating his own lightsaber, he leapt up into the rafters of the building and diminished his Force presence, hiding in the shadows.
Being a secret apprentice had given him a knack for hiding when he wanted and he wanted to see if Mara could find him with him not wanting to be found. He had a feeling she could, but he wasn't going to make it easy on her. Slowly he made his way to the other side of the large space, being careful so as to not give away his position with any kind of sound or feeling in the Force.
Galen kept his breathing quiet and his eyes focused on Mara as he hid in the shadows. Finding a comfortable spot where he could crouch down, he waited to see what Mara would do next.