For an easy way to beat the flood, as an Elite turn on your cammo(white button) and then sneak up behind them, and use the strongest human weapon you have, shoot them in the lower back (where the stomach is, because that is where the parasite is) or in the back of the neck, then run like crap and hide becaue you lose invisiblity when you shoot, then do it again. (the sentinal beam works just fine, it just leaves you open longer, so shoot from farther away, very simple. this is time consuming, but it helps when you are playing by your self.