Oh, and for my review. I love the graphics and how sometimes your shot misses even when your aiming very close to it (not sarcasm). The weapons are awesome too. but i could do without the quite slow gameplay when you are not on nightmare, which I found quite hard. 5/5 though.
Not actually done the achievement yet, but I am pretty sure that a good way is building fake walls and leave only one gap, so they only have one way in. Just rapid fire with anything, even the pistol.
What an amazing game, I don't give many but 5/5. Not grinding amounts of gameplay and the art is amazing. Such subtle differences too, it's even better since the differences change the times you play. Quality.