I haven't tried Veteran, but on Hardened I made it through this mission without a scratch, using maybe one magazine and no grenades or flashbangs. I played it over and over on Regular and found it really hard, but that was when I was trying to "complete" the mission in traditional shooter terms by killing all the tangos. Because of COD4's spawning enemies, the key is just to move counterclockwise through the doorways on the first floor until you reach the hallway that leads to the stairwell, then defend yourself until the rest of the platoon catches up (it appears you have to kill a handful of enemies before your team will be ready to breach the door to the second floor). You really aren't required to kill much of anyone on the second floor - your team can handle that work for you. There's really no point in firing a single shot before you make it to the large hallway that contains the door leading to the second floor.