08 Jan 2010
Oddly enough, this is easier in single player, since you get more checkpoints, closer together. Personally, I did the whole game split-screen co-op on medium, and did the game on extreme solo, and my solo playthrough was easier.
Knowing the whole game helps a lot. Just take your time, use the sniper to kill enemies from afar (like in the snow level) and the machine gunner anywhere else.