Beating the underwater level where the wall comes to squash you is all about timing. They move at the same speed no matter which difficulty you are playing on. When I was new to GOW I had trouble with it and hated it but after beating the game 76 times now I don't even blink when I get there.....
You have to start using your underwater thrust as soon as you come down. Get into the first hole and stay as low as possible. When the wall comes it a case of just timing it so you can come up without getting hit, thrusting foward TWICE to get to the next hole. Center the next hole ASAP and dive down. The level is cake after that....