First of all White Shadows has a lot of good tips. Now let me put up my guide:
First of all, use the Riot Shield/Desert Eagle combo ASAP. Second; ALWAYS USE COVER AND YOUR TACMAP(hold back button) There are a lot of ambushes in the game especially in the Casinos. To try and get around this(still REALLY annoying) is to send your team toward an objective and bring them back to you which will hopefully trigger the ambush, and while doing this look at the tacmap to see if they spawned and also not to be flanked.
On to the Level Tips:
First let me say Im only going to list the Parts I had a lot of trouble with, I cant provide much help but I will try.
Mexico: The only hard chapter for me was "Mines". What I did was put my brightness up to 9 so you can see the people a little easier, use a silenced pistol on the first 2 enemies. After that I picked up the shotgun killed the first guy I saw down the hall and then took cover on a crate and killed everyone I saw there then looked at the Tacmap and cleaned the last 1 or 2 guys up. Then as you continue there should be a guy hiding behind cover blindfiring and occasionally peeking out. you can either wait or just go around carefully and just shoot him in the back. And for the last 3 guys you stay to the very right wall and shoot that guy then back up, peek out a little bit(Be VERY CAREFUL HERE AS 1 SHOT WILL EASILY KILL YOU) and shoot the guy anywhere hopefully killing him and then peek out and get the last guy and move on. I know you may need to memorize the enemy positions and stuff, but thats ok I was stuck here for about a day and a half on and off. This is also the only time I recommend using a silenced pistol.
Calypso Casino: "Prime Time" was ridiculous to me until I realized little things to make your life easier. first of all, after killing the enemies on the first and second floor just go into the little maintenance room that is near the main stairs and then go to the third floor and if you go out of the right door if you keep hugging the wall you will find a little area that will allow you to pretty much take out all floors of enemies. You will most likely need to send your team to the stairs leading to the fourth floor to trigger the ambush that spawns the enemies.
Dante's Casino: "Data Hub". All im gonna say is wow. I didnt think I would EVER beat this level. I cant even really tell you what to do. All im gonna say is use the riot shield/desert eagle and just hope your team doesnt go down because it seemed like if they ever went down, the enemy would trick me into trying to pick up my teammate and then killing me. Watch out for this level or maybe its just me that hates this chapter.
Nevada Dam: Personally I didnt think this level was tough at all until the very end which still only took me a few tries. You just need to be prepared to have your team in a spot safe enough to keep them alive but close enough to the enemy to let them kill for you.
Overall I found this game EXTREMELY hard on realistic but even I managed to get through it... eventually and Im not that great at shooters. You need to be really really patient and also find a gun that works for you(my set up was the riot shield/deagle but not everyone likes that). One last thing is that if you ever feel really angry with the game or feel like you wont ever beat it, just go play another game. For me I just play a little guitar hero or help someone in a game and then try again and then chill until I finally beat the part. I hope there is someone out there that was helped by me. I just got the achievement 8/29/10 so its not like Im posting this like a year after I did it. I do have a vid of the achievement unlocking for me but it doesnt help anyone just to see it so I cant post it.