This is a harder level. You never run out of people coming out of that door. Diplomacy is a spawning level. The best thing to do is get to the other side of the building where the glass is after killing everyone that you can (go straight towards the little awning that you see at the start of the level and kill everyone until you are no longer attacked.) When you go inside, you have to becareful of all the militants coming in. I would suggest you fit all your guys with med packs because you will NEED them. Once you get upstairs, you once again have to fight them off. Once you get to the large room, you need to throw a smoke grenade and get out to the next floor asap. That is the part that the game also enjoys spawning militants in. Other than that, think/be TACTICAL.
uuh umm diplomacy was a also hard lvlt to beat for me but wat i did was let two people cover one door and let the other cover the other or stay in a group behind cover like wat gamble sarts said i dont play anymore its wack becaus ei hardly see any body online.