The FASTEST and Simplest way!
I'm am not sure exactly which level you receive the move, but the only requirement is the move called Knee Drop :)
Even with another move substituted i.e, lariat (slow and clumsy)
this method is by far the fastest.
The more inventory you can hold speeds this up, I wouldn't try at level 1, lol.
I used the method where you wait the half day on the roof, and pass the missions up, giving you extra time for other cheevos after.
Once you've entered Paradise Plaza stay there.
1.Grab the pies from Roastmasters, then go outside.
Back in and grab more pies, maybe repeat once more.
Back upstairs to use the blender;
1 Orange Juice + 1 Pie = Untouchable
2.Fill inventory with 'Untouchable'
3. Knee Drop the shit out 'em, lol.
You'll notice they try to spawn the zombies "randomly" with code,
you will see patterns of scattered groups.
Find a group, go stand in the middle, drink 1 untouchable and Jump then Knee Drop, or Lariat or whatever.
They can't touch you (occasional punches don't do shit)
4 Every time Untouchable runs out drink another and repeat.
By Far the quickest way!
1hr or so if ur good :)