Start a four-player local game, with you controlling all teams. With your main team, move one of the worms into a dip in the scenery, and throw a water balloon or call in a water strike to that location. Make sure that at the end of the turn your worm is underwater.
Now, let the other teams have their turn and make sure you don't disturb this first worm. After five turns (so once all teams have had one turn and two teams have had two), your original worm will have been underwater long enough and the trophy should pop.