Also grapple the non brute germans and push them out the window you broke coming in to the room. these are quick kills. Start with those near the window. whip the brutes at the neck. keep moving. eventually you will have one brute left. keep moving, Lure him near the window with his back to it and one good swing of the whip and he's out window
I think you'll need to kill/disable at least 10-12 Germans and possibly release the 2 fire extinguishers under the walkways. I haven't heard of any major glitches with this scene, so hopefully you'll be able to progress.
Best of luck.