Both methods are shown in the video.Method 1:
Load rooftops in Echo mode, run into the first room, drink the nom juice, and then shoot a flare at the cieling. After it explodes, hold LB to use your thumper, killing everyone in the room (hopefully) and getting you 2100 points.Method 2:
Get to the elevator track without killing the green zombie. Allow time for enemies to bunch together on track. Kill the zombie, while he is close to you to get infected, then shoot a flare above the enemies on the track, and use the thumper (LB) to kill all of them, netting you around 2900 points.
See the video below.