What I did was I camped between the tower that's near-by and the ledge. The EDF only comes at you from the same two places, just whip out the nano-rifle, zoom in and pop them as they come at you. Doing that I had no problems with Sam or the buildings. Just keep an eye out on your radar and melt them when needed, or if you're using the trainer you can turn on the matrix mode to give you more time to decide what to do. I use the matrix mode on those gunning for Jenkins missions, lets me blow up so much more stuff.
Just wish we had an option to kill the timer on the timed missions, my driving skills suck sometimes.
Thanks! Matrix did the trick! Also the message goes out at normal speed when your in it so it makes it even easier to complete!