Alone it's easy, IMO. With other people (who aren't competent), it can be painful. It took me a couple tries but after a while you know where to stand/what to do. Plus, once you get the rocket the game is pretty much begging for you to die :) You aren't picking up that crap-ass spread shot are you? The only weapons you should use are the standard machine gun and toss that when you get the rockets. If you want a real game that drives you insane, play 1942: OY.
er, too bad you asking on PSN :/ I'd be willing to help you sometime on Live if you need it though.