Also known as the "Real Mega Man Blaster". Here's the easiest way to get it.
To do this your going to have to kill 53,594 flesh eating zombies, so start a new game (don't bother about the case file missions and let all survivors worry about them selves... yes I know they'll die but your reward will be worth it).
Head outside as soon as you can then head between the food court entrance and the north plaza (maybe closer to north plaza) and you'll run down an allyway into a carpark (note: only bring food your not gonna need a weapon).
The carpark will be swarming with the undead so run for the red sports car, the zombies wont harm you when you're in it, infact its quite the opposite!
Drive your way into an underground tunnel entrance that's further into the carpark and run down all zombies in your way (mote: like all weapons the more its used the closer it comes to breaking).
The underground tunnels are flooded with zombies so fly through the crowds, if you go north to a dead end you'll see a van you can use when your redcar breaks and if you drive back into the carpark a new red car will have respawned
Keep swapping your cars often so you never have to run through an army of monsters (dont worry I had to run for it loads of times and did this achievement no problem).
You can go back to the mall and save your game or stock up on food when you like, I didn't and killed over 54,000 zombies within a day and a half.
When you complete your target finish the game any way you like and when you start a new game the real mega blaster will be in the security room (the part Otis is in).
You get 300 rounds and 2 hits in the face will kill most bosses, 1 hit kills zombies.
Every time you go back to the security room drop your blaster on the floor before you go through the vent shaft or the weapon wont respawn.
There'll be no need to learn the "zombie walk" - its time for them to fear
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