Weapons Expert http://www.wpcentral.com/sites/wpcentral.com/files/styles/la...You must use every available weapon or tool in the game while in matches, it is cumulative and doesn't have to be in 1 game.
You can do this in a local versus match using your guests with your controller.
Once in a match (Any open map, Full Wormage game mode) just start using all weapons and tools along the list, you do not need to hit any worms in the process.
Some tools that you might have a question on how to use:
Worm select: Press A then Y to switch worms
UFO: Press A on a girder to move it
TeleSwap: Press A then Y to switch worms
Teleshield: Press A then Y to choose worm, A to activate
Kamikaze: Will suicide the worm (recommend for 2nd last)
Surrender: Will forfeit your team, (Use last)