Army of Two (EU)

This mission is reasonably easy, until the last section. If playing with AI, play cautiously, always keep the AI near (the death timer is very short on this difficulty), and go slowly. It will take a few goes, but it's not especially hard.

The problem is right at the end. There is an hellishly difficulty back-to-back section, where multiple units come in from all sides, from multiple levels and often have cover. This section is also much, much worse with the image distortion blackout effect when you take damage.

I'd give it a few goes, try swapping a fully kitted PKM with the AI to help it out. If that fails, you will probably need a friend.

As mentioned in the other two solutions the back 2 back is the hardest part of this level but what I found easier was to use Rios instead of Salem. I had many attempts playing as Salem and due to the spawn doors being further apart it meant you have to move more stopping the AI partner from being as effective. Also having a shield added to the front of the gun seemed to help me a lot as I didn't seem to take as much damage. Other then moving to kill the enemies coming out of the door the only time I had to really change position was to finish off the last enemies in the middle of the room. Although I did go down there wasn't enough enemies left that the AI could revive me no problem.

This level is more or less a cake walk, unless you'll rush ahead like crazy, but if you stick to cover and use agrro to your advantage things shouldn't get too hairy. The back 2 back section on the other hand is a bitch, but definitely do-able solo. I've tried many guns, started with the shotgun, then switched to the granade launcher, but to my surprise the gatling gun turned out to be the best choice, got it on my first attempt.
One more tip: bear in mind that you can change your equipment by poping out to the main and continue from given checkpoint.