Both of the other solutions are correct, but they fail to inform you of how to actually create a custom game in order to change weapon settings etc.
1) From the main menu go to Versus -> Classic (any mode counts)
2) Press A on 'beginner' under Game Settings
3) Scroll to the bottom of the list and hit 'Create Game Settings'
4) Enable Donkey (bottom row, 3rd last) by pressing LB to increase ammo
5) Save game settings, back out & add player > clone of [self]
6) Ensure 'teleport in' is ON for ease of moving worms about
Once in, teleport all of your worms to the same kinda location and then hit B and launch the Donkey weapon. I literally stacked them like cards, haha.
This set up should also net you:Extra note: for the landscape I chose 'beach > one island' and randomised until I got a preferred layout to make this achievement as simple as possible.