We've shown videos of hornets attacking and killing bees before. If you haven't watched that video, I recommend you do, it's amazing.
Japanese Honeybees, however, are not so helpless either. Scientists have observed an interesting technique they use to kill the hornets. If the honeybees manage to surround the hornet, they'll form a bee ball.
The honeybees' stingers can't penetrate the hornet's shell. Instead, they vibrate their muscles which causes the temperature inside this bee ball to reach 116F (47C), which is enough to heat the hornet to death!