Worms Battlegrounds

Achievement: Behold!

Go into local -> versus -> deathmatch and add as many guests as you want (add more if you're gunning for other achievements). Use the 'Family' game mode setting.

Crates will appear almost every turn, you will have 90 seconds to collect them and you start with a crate drop, which gives you 5 weapon crates straight away, 20 if you have 3 guests dropping weapons for you as well. Simply get and use one of the weapons on the bottom row of columns.

Behold!

1)Go into a local versus Deathmatch
2)Add a guest player.
3)Set game mode to Elemental.
4)Select open map.
5)Kill an enemy worm and keep collecting the crates appearing until you get a Legendary weapon (Any weapon on the bottom column of weapons).
6)Once you obtain 1 of the bottom weapons just use it and the achievement will unlock.

This achievement can be earned in the single player Campaign mission called "Pre-Zen Break."

Pre-Zen Break is either the 16th or 17th mission in the campaign. On the map showing all campaign missions, it is just to the right of the golden temple in the middle of the map. The path splits after the temple, you want the lower of the two options (the one on a small island).

When you begin the mission, use a Firepunch to kill the guard and open the prison cell. Then follow the path to the right, and you should see two crates with a sleeping enemy worm in between them. The first crate contains "Stick Up" (which allows you to steal weapons from enemy worms), and the second contains "Gravedigger" (which allows you to steal weapons from enemy gravestones).

Return to that sleeping worm, and use "Stick Up" to steal a weapon from him. You can do this multiple times to steal multiple weapons. At a minimum, I recommend the mine and bazooka.

Now use whichever weapon you stole to kill that worm in one hit (the mine and bazooka can both do 40+ damage). It is important that when the worm dies, its gravestone remains on the screen and doesn't end up in the water; so I don't recommend using The Banana Bomb, Holy Hand Grenade, or any other legendary weapon you stole from the worm (despite how tempting it might be.)

Now move your worm over the enemy gravestone, and use "Gravedigger" multiple times. Each time you do so you'll get a crate, and it is usually a legendary weapon (listed in the bottom row of the weapon select screen).

Now, just use that legendary weapon and the achievement will unlock. It doesn't matter whether you hit anything with it.