The other solutions here are great but the game has been patched to allow you to do this in the tutorial, so the above guides are no longer relevant. The tutorial now spawns a Gumball gun instead.
*** Important note regarding the Xbox 360 version***
The 360 version of the game isn't the same as the Xbox one version. In the 360 version of the game, the trampoline area isn't fenced off if my memory serves me right. You can still round up the zombies in the bar area close by.
Here's what I did, but there is prep work and rng involved.
Mutators: Double Jump, No Hunger Goat, Casual Goat, Angel Goat
Spawn in to Goatz post outbreak, you will spawn in the street. Immediately turn around, at the end of the street, off to the left is a garden type area with a trampoline. Inside will be 4 workmen - Press
on every human enemy from here on in to infect them , and get them into this garden area using
to lick them and pressing
to release them when you need too. As well as infecting humans, make sure to move as many zombies into this area using the method above.
Crates will drop from the air containing supplies. Press
on a crate to open it. What you're looking for is the Minifun - a machine gun. I can confirm that the spawn of this weapon is heavily rng based. There are multiple places around the map that these crates drop, so just keep looking at the sky to see where one is dropping.
Once you have it, equip it by holding
, and assuming, that during your downtime between drops, you've been rounding up zombies in the garden, you should now have a garden full of zombies.
to kill 7 zombies with one Minifun.
I tried this for 2 hours one night and upto day 36 of the outbreak - no Minifun.
Tried this a second time today and had my gun by day 3. The rng isn't even funny when it comes to this achievement.
Good luck and I hope this helps at least one person