To make an essay with only short time, first, make an outline about your topic. Then, you can start writing freely by just following what you have outlined. And put what's coming out on your mind. :)
First, UNDERSTAND the topic
Use a imaginative mind
Make those "thinking-time" fun
Don't be too much serious but know the limit
Just enjoy what you are doing then have an inspiration that is somewhat related to the topic of your essay