I have a feeling you managed to do that on the bank map, if not, you found a good spot on the bus stop map.
I was trying to beat hard, on the bus stop map with a shotgun (which I used like a Winchester), my problem usually comes with killing the blimp fast enough,
The game slows down before I finish my task.
I tried to use smoke grenades, but with the amount of enemies, i lost accuracy and missed some spots.
Thanks for your advice!
edit: In the end, I finally manage to get it.
In some parts of the wave 10, I got a constant 4 fps...
My strategy was the same one used in Doom. Run around the object.