What you need to do is find two sweet spots in the final room. Close together. Easily move between them.Stay in one and shoot the fuck out of the brain. When the cells are about to hit you, move to the other spot and then shoot form there. They keep their initial trajectory, so just wait until they have all fired and then move. If you move while some have no fired yet, some will go going to your new location. So you want to stop move stop move stop move, instead of continuous movement. Once you get it down, its easy.
