This is not too difficult to get.
Choose casual difficulty, then load the last part of the first level (the showdown in the restaurant). Run upstairs, jump to one of the lamps and keep hanging on. Look around with the right stick and shoot every enemy...keep hanging and wait for the next wave to come, shoot until you run out of ammo. Jump down, grab a weapon and repeat...you might have to play the level twice or three times until you have killed the required 50 enemies.