The game itself is not that hard, there's the odd hard level but nothing too bad. It does take a while though mainly because of the No, Seriously achievement (10,001 kills). If you have an autofire controller you can set up an automated process, you'll either need a boosting partner or a second console/copy of the game to do it.
In terms of difficuly I would say 4.
It took me about 4 days to complete the single player stuff, playing probably 3 or so hours a day. All multiplayer can be done in a few hours, apart from No Seriously, which took about 30-40 hours (with 2 copies of the game and the autofire controller)
Last edited by Joel78; 02-13-2012 at 11:35 PM.