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