Completion Strategy:

If you're thinking of getting platinum on this level, there are a few things you'll need to learn.
First of all, there are 2 methods of scoring.
You can use the Shotgun, and score 75 points by shooting off the zombies head, or 25 points by shooting anywhere else but the head.
Sometimes you can score 75's and 25's at the same time, if your shot is lucky.
Another method of scoring is good ole' fashioned punching.
One punch to the head is worth 100 points.
I personally do not recommend using the 'punch the head' method as it's critical to obtaining perfects.
You get Perfects by going through a single wave without getting hit.
If you go through a wave without getting hit, you'll get bonus bonus points, which I believe is double the amount of points you scored during the actual wave.
Another thing is use manual aiming.
If you've seen my FOTLD video, you'll notice that I always have the aim cursor on at the zombies head height,to gain easy 75'ers.

Now, probably the most important rule of all is, DO NOT LEAVE the actual room you spawn in, AT ALL.
If you leave via any of the 4 exits (including the stairs), you'll only have 3 seconds to come back to the area.
You've learnt the basics now. On to the actual strategy.

Platinum/Fast Time Strategy:

Because of the fact the Waves last much longer as you progress, you'll need to be very conservative with your ammo, especially after Wave 45.
That's when the real pressure comes in.
But I'll start from the beginning.
During the first Wave, the zombies will only be coming at you from the middle entrance.
You can either shoot them from where they're walking, or let them come to you, and get heaps of 75'ers.
Either way, it dosn't matter, as long as you don't get hit.
During Waves 2 and above, the zombies will start spawning from not just the middle entrance, but in the room on the left and right.
What I always do, is focus on the room on the right, then the room on the left, then the middle entrance.
Sometimes, you just have to guess or assume if there's any zombies coming for you from another direction.
If you're still alive up to Wave 45, you'll need to be extremly cautious with your ammo.
If you performed the average misses, you'll run out of ammo a few times, like I did.
What I tend to do is keep on killing the zombies until I'm up against the last one.
I just keep on walking around in circles, until the weapon spawns, and again, to gain more ammo.
Either way, getting above 145,000 is cake.
Yes, I'm using Neo's term. :p.
My score for this challenge is 1036100.
That's right above a million.
It took me around 4-5 hours to obtain.

- Lenny