You guys are talking about the Favela level right? With all the houses and mexicans killing you while you have to capture Rojas?
Try that level on veteran. It really isn't that hard. You just have to really learn how to play the game in those situations, you can't just run and gun, cover is your best friend!! On veteran you kill 50 and 200 come back, and you get killed in 2 shots, lol.
The BEST thing to learn on COD, or any shooter for that matter, is that the enemies keep coming no matter if you keep killing them. You have to advance your way through the level, no matter how many you kill, there will usually be more, just keep working your way through the level.