Worms W.M.D

It's very likely you will unlock this naturally. Campaign mission 'Tanks for the Memories' is a good place to try this as there are two health crates near the start and you can easily kill every worm from a distance using the tank.

Otherwise you can try this in a local match. Set health crate drop to 100%, pick one up using one of your own worms and just have the other human controller team surrender.