I just went straight to veteran, and I found it helped to take my time on most of them. You could also check out some youtube videos for some tips, if you dont consider that to be cheating.

coop is the best route.
the 3 levels with juggernauts are an absolute must.
 
On the level with you and 15? juggernauts and only explosives it is simply required you cheat I've yet to meet any one that can do that coop or solo without doing some sort of cheating, and I've damn near played that mission over 50 times before I cheated it out.
on the oil rig one person the one with the .50 cal setsup at the first scarfolding you come across its before the first hostage room, the other person leads the juggies to them and that person takes them out.
 
I can't remember the names of the levels but the plane graveyard the hardest in my opinion that you can't really cheat at you simply have to keep it as a nice pace don't go fast makes sure you kill every one, set up choke points once the juggies start appearing after you slide down the kill cause they will kill you very fast indeed.