I suggest doing the campaign in Hard mode, I heard it was too easy otherwise (AI). I didn't have too much problem completing it, aside from 2 missions that were harder: The shooting range part in Kaffarov and the mall defense/extraction
I finished it on Hard yesterday and I did get killed a lot during the first mission although that was probably more to do with just getting used to the game and figuring out how many hits you can take before you die.
I found some good cover spots in the mall section so that didn't cause me too many problems but the very end did when you have to run to get on the Osprey with the rest of the soldiers. I couldn't see where the RPG's were coming from and must have tried it about 7 times before I got it . The Kaffarov shooting range was stupidly difficult due to the enemies all having those blinding laser pointers on their shotguns and also the 1 hit kills
Some of the trophies are badly glitched too. Took me 2 attempts to get the one on the first mission to ping (protect the wounded soldier) even though I done it perfectly the first time. Took 5 goes to do the timed part on Comrades and then 4 goes to get the one on Rock and a Hard place for taking the Jet out with one shot.
I wanted to get all of the single player trophies but gave up on the one where you are in the jet. Got all the rest though
Co-Op mode is frustrating too because your partner is either a moron who stays too far back and lets you do all of the hard work or else they drop out of the game before you can finish the section that you're on