Important: you will have to have flood and freeze and it will have to be a multiplayer game yourself versing yourself.
First select a map that has a edge thats surrounded by water (for example coloseum of doom). Second get a worm to go to a edge thats surrounded by water.Third when it is that worms turn(or just use worm select) open the weapon panel and select freeze. Fourth when it is the enemys turn make him do flood and the ice cube will still be there but the worm wont be there!
Unlockables
Best Defence: finish mission 17
Check Mate: finish mission 8
Construction Junction: finish mission 6
Monkey Troop: finish mission 5
The Blue Dragon: finish mission 13
The Great Wall Brawl: finish mission 11
The Question Of Faith: finish mission 19
The Ship Of Fools: finish mission 14
Tower Of Power: finish mission 16
Under Construction: finish mission 3