On the third part of the Jerusalem stage, once you go down into the underground base all of the walkways are metal gradings, most of which you can go under. This is very helpful since there is a lot of people in a small area, you can just shoot someone and duck under the gradings. Even if they see you they can't shoot you through the gradings. So you can hide until they forget about you and keep repeating the process until all the enemies are dead.