A person who is having a temper tantrum is generally experiencing extreme feelings of frustration and anger. They obviously do not have complete control of their emotions and may not have the current ability to act in a rational manner.
Your best defense in dealing with an aggressive individual is to remain calm. Becoming aggressive in return will most likely only increase their frustration and anger.
When the person has regained their composure then that is the time to discuss the reasons for the temper tantrum. Terms such as, I understand your frustration, and I understand why you could be upset, will help to allow the person to discuss the frustration that brought out the temper tantrum.
This is the point when it is generally safe to express your feelings in the situation and to try to come to a resolution that will help to end the aggressive behavior.