As stated above, finish the last day of '7 days of hell' mode. this mode unlocked for me during my second co-op playthrough at about day 25.
I'm playing this solo and it isn't nearly as bad as i thought it would be.
You can play these days one at a time and use continue as many times as you need to to complete each level. I played the first 3 in one sitting as it didn't take that long but from then on i would recommend doing one day each time you turn on your xbox as it can be a grind.
at the end of each level, make sure to let the game tally your score and save BEFORE turning your console off!
Day 1 - 10-15mins / 1400 zombies - Stay in bottom right corner by the digger with back against the fence
Day 2 - 10-15mins / 1800 zombies - Again, stay in the bottom right corner near the helicpoter blades with your back against the right wall
For the above two days dont worry about weapon drops, just keep your back against a wall and use the machine gun as they wade towards you.
Day 3 - 30mins - 2200 zombies - you can't really stay in a corner on this one so use the wood chipper and conveyer along the bottom to kill scores of zombies. this level introduces Big Boys so you need to move about when they show up otherwise you will die alot.
Day 4 - 45-50mins - 2600 zombies - this is where it starts to get tricky and very annoying. knife-throwing old ladies show up in this one as well as the previously mentioned big boys. again, keep moving and use the car crusher at the top to your advantage. you will die ALOT on this level because of throwing knives!
Day 5 - 50mins - 2950 zombies - this day inroduces the shotgun wielding cowboy zombs, they are v.easy to kill as they're slow and have limited range. again, it is the knife throwing old ladies that will piss you off on this one, best bet is to keep running laps around the edge of the map. there isn't anything on this level that you can use to kill zombies (ala, above car crusher etc.) but the cowboy zombs do appear to shoot other zombies whilst trying to shoot you!
Day 6 - 45-50mins - 3250 zombies - this day introduces the suicide bomber zombies. however, this is a good thing! they can be used to kill other zombies and as long as you deal with them promptly they are not that bad. again, keep doing laps to avoid knives and use the petrol pump to burn the f*ckers down!
Day 7 - 80 mins - 3800 zombies - this day has all previous zombies and introduces the pregnant, spawn-birthing c*nts. KILL THESE A.S.A.P, they will keep producing spawn until dead and the spawn, when killed, do not add to the total of kills (they are basically there to drive you nuts!). for the earlier waves i stood between the pick-up and the trash-compactor near the top right, you can pretty much chainsaw anything that comes at you.
Glad i finished it, won't be playing this game again in a long time after this cheevo!