If you're using the pistol provided do as follows:
- don't shoot the loan wondering chicken (which spawns near you), save him for if you miss a couple of shots after the mass chicken wave to keep you're multiplier up
- follow one of the humaniods, keeping your croshair about a meter ahead of him and pop the chickens off before he grabs them.
- don't waste bullets on the mass chickens, just kill 1 or 2 to maintain the multiplier
Or you can do it the 'cheating' way...
Press start, switch loadout (y) and exit.
Then just kill a few chickens, wait for big wave of chicken and stick a grenade on the floor .... Couple of grenades and you're get gold!