HAVEYOUS33NCHEF has a great guide if you have the Megaman Blaster and make it past 72 hours mode in the story.... BUT to hit level 50 on your first playthrough the soonest possible time
this is the guide you should use.
Once you enter the first area outside the security room (Paradise plaza) head right to a store with some bookcases (can look at map and look for store called Bachman's Bookporium) and grab the book Horror Novel 1 which will increase your PP bonus from killing zombies by 25%.
Now play through the first main story mission and get into the Entrance Plaza go to the 2nd floor right side facing the Mall main entrance to a book store called The Sinister Read and grab the pp book Horror Novel 2 which will increase you PP bonus from killing zombies by 25%.
With these 2 books head outside towards the middle point of North Plaza at Wonderland Plaza (shows a car symbol on the map) and grab the red sports car, you will want to head into the garage and start killing zombies. Using this method and having these 2 books will net you 750 exp for every 50 kills and 30,000 Exp for every 1,000 kills (45,000 exp per 1,000 kills with the 50 kill bonus's). If done correctly you can kill 1,000- 1,300 zombies every 5 minutes.
You need a total of 5,000,000 PP but using this method you will get this achievement much quicker than you think. Also while doing this method you will obtain the Mega Man Blaster once you finish the playthrough with 53,594 zombie kills.
For an more in depth look at the pattern of killing the zombies check out the Zombie Genocider guides:
You will also obtain these achievements along the way:
Note: You will not get the achievement at 53,594 kills but keep doing the method after the achievement until you hit level 50.