All of them except Naughty Naughty and the Firefight point achievements, I have the ones for Alpha Site Interior and Lost Platoon, need the other ones though.
I just generally played through the game for story related achievements, went hunting with a friend for the Audiophile achievement, while ones like Saint I did a run through of campaign on my own to get.
The achievements in this were quite easy, only missing the ones I am because I haven't been to bothered to try getting them, although the vidmaster ones did annoy me a bit, Classic wasn't to bad, Deja Vu was mehish, Endure killed a part of my soul however.
Not because of the difficulty however, it was mainly because when I first attempted it I got to set 4 wave 1 round 1 when my live messed up major, got it with no interruptions afterwards though.
Credit to .Impact for the sig & avy