Well what I did (I cheated) I used the ( giveusatank ) cheat. and stashed it right next to the car and as soon as all the psyco guys start comeing i jumped outa the car and into the tank. and just ran'em over and used the tank to blow up the vans. Mission passed.
What I do is run out of the parking lot through the north entrance,(killing a bomber and the van that are is that direction) then once I get toi the street I turn right and a litte ways up the street is a stair way that lets you look down on the parking lot you just came from(this stair also leads to a hidden package) the from the platform up there i destroy all the vans and bombers.