My brother, two random people, and I got 12 mins on No Mercy's Gas Station. Have everybody grab molotov cocktails and auto shotguns, then go activate the lift. Stay on the lift and run straight to the warehouse without stopping. Break through the window and go down into the hole. Have two people over on the pile of shelves or whatever they are in the corner next to the pill box, one right under the hole (infected coming down will jump down in from of the survivor and will be easily dispatched), and one between the closet and the door. Have everybody use pistols until the tanks come, and then unload with the shotguns. Use molotovs when the horde is too big during tank attacks, or when there are multiple tanks.
You don't need to worry about smokers pulling you from out the window, because AI smokers come onto the pipe right outside the window rather than staying on the street.
Tanks will always attack the person in the very corner on top of the shelves, but if everybody is focused only on the tanks with the shotguns, they will go down before any survivors are hurt.
I just copied my post from another thing so it sounds like a tutorial, I'm not trying to sound like an ass or anything.