This is a really tough one if you don't get the ending right. Most of the level is pretty straight-forward; enter rooms quietly, identify the two hardest targets and mark them, then go in with your team and attack. There are sections in which you can sneak past the majority of the enemies and avoid firefights, once you reach the theater portion of the casino (good for CONSERVING YOUR GRENADES). When you get to the theater, slowly work your way around each balcony level, taking out any enemies, while keeping an eye on the stage, and returning fire when they spot you. Once the theater is clear, regroup with your team, and head to the left side of the stage (stage right :P) and enter the door just before the orchestra pit. Head down the hallway and take out the enemies in the furthest section, head back toward the opposite side of the stage, and watch for an enemy hiding behind a crate on your left, just before you head into the last corridor. Once on stage, set your teammate to hack the terrorist computer, and set your team to ASSAULT and send your remaining teammate to the crate to the LEFT of the computer, while you take position on the RIGHT, behind the turret (it's important to get all of this ready before the enemies come in).
Once your intel gal chimes in, there will be three waves of opponents. The first will be pretty easily dispatched, be sure to take most of the targets on the left, and your teammate will get what's on the right. If you're having difficulty with things in the center due to smoke, use your thermal optics, and if necessary, mark the targets, because your teammate has deadly accuracy, when he chooses to fire.
The second wave is probably the most difficult, as it is the longest. Pop smoke in front of the computer, maybe a little to the left, so the smoke doesn't block you. Take out a few snipers in the balconies, and the swarms of forces on the floor, and make good, short bursts on the enemies in the seats. If an enemy is hiding, don't wait for him to pop up, mark him for fire and move on to the next enemy. Once you finish the last few snipers, the third wave begins, which will include fast-roping enemies. If you can, try to grab these guys before they hit the ground, and send your frags out down each aisle (you've still got frags, haven't you?). Smoke your computer once more, and now it's time to warm up that turret. With the turret, the third wave should be quickly dispatched, be sure to watch the floor vigilantly, mark snipers for your teammate, and after a bit of silence, your intel gal should tell you to proceed to the roof.
YOU'VE MADE IT PAST THE MOST DIFFICULT PART OF THIS MISSION, HIGH FIVE!
But wait, you still have to evac. Unfortunately, you PROBABLY WON'T get a CHECKPOINT after defeating those enemies, so proceed with the utmost caution to the chopper. There will be a ladder on stage left, where you positioned your teammate, send your friendlies up this ladder, and follow them after you hear firing diminish. There will be a catwalk to your left, send your squad all the way to the end of the catwalk, in assault mode, to take care of an enemy or two in that hallway. After this, have your teammates open and clear each door, and just hang back, until you successfully evacuate.