The Jellyfish that ate British Columbia
Canadian nuclear testing and industrial pollution off the coast of British Columbia causes the simultaneous mutation of several hundred jellyfish into monstrous size and also allows them to crawl on land. They immediately attack several fish farms, and Victoria. Professor Madfinkelstein, his beautiful daughter Helena and his dull but able assistant (Joey) are called in to deal with the jellyfish threat.
At first, they attempt to make the jellyfish retreat to the ocean by waving brooms at them, and making "shoo" sounds. The jellyfish retaliated by giving them all lots of painful stings, and making them spend the afternoon soaking themselves in vinegar. The military was eventually called in, but they are helplessly outnumbered by the jellyfish, and many men are eaten as a result.
The jellyfish are destroyed when Professor Madfinklestein unveils his "frequency destroyer", which kills them by emitting sound waves of a specific frequency. 
Ebert called this movie "a triumph of the soul over giant, stinging jellyfish".
Comic relief is provided when a small annoying boy and his small annoying dog (Ruffy) are consumed by jellyfish. Initial scripts had Ruffy surviving the attacks, but moviegoers called the character as portrayed "disturbing" and "insufficiently anthropogenic", and so it was dropped. It's part was performed in the sequel by a medium-sized rock.