What is a "reaction" piece about the movie Life of Pi?
If that is the writing prompt from your teacher, then what you need to do is write a paragraph or two about what you thought about the movie. It would be great if you have read the book too, because then you could have a paragraph that describes ways in which the movie held close to the book and ways in which the movie did not follow the book so well.
You could spend some time writing about the cinematography of the film. It is a beautiful film to watch with all kinds of bright and vibrant colors. How did the scenery in the movie make you feel?
You might want to respond also about how Pi and the tiger's relationship made you feel. Maybe your feelings changed. Were you scared for Pi at first? Then what? Maybe you found yourself relating to Pi in certain ways. Write about that. Could you have survived like he did?
If all that your teacher is asking you to do is write a "reaction," all you need to do is express what you thought about the film and why. That should be relatively quick and easy if the minimum word count is 100 words. That is ten 10-word sentences, and chances are your sentences are longer than 10 words. Pick three things that you liked or didn't like about the movie or characters. Writing 34 words about those three parts should be manageable.