I've tried this multiple times and we always clear out all the zombies. Does it matter if you have 2 players? Because I had my brother throwing attraction phermone and still no zombies were coming after 10 minutes of killing them.
On another hand, I found the second level very easy to farm XP. Use Spectre and get all 38 collectibles (there is also a challenge to use the computer, so you don't have to destroy the servers that gives you 200 xp) and shoot all 20 cameras and you can get 12,000 xp in 15 minutes.