You're on your own writing the paragraph, but I can help you with the basic idea. I'm not sure that protagonist is the word I'd use with Puck simply because MSND has many characters that could be considered protagonists. I would call him the central character, though. The central character of a Shakespearean play is the one character that ties together all five acts and their relation to the Elizabethan Five Act Play Format: Act 1 - introduction, Act 2- Rising Action, Act 3 - Conflict/Climax, Act 4 - Falling Action, Act 5 - Resolution.
Puck is really the only character that we could make a strong case for as the central character. His main weakness is his exclusion from Act 1 (introduction); however, he is introduced immediately in Act 2. He is then the only character involved in the rising action of each plot:
1. He accidentally confuses Lysander and Demetrius.
2. He gets the flower so Oberon can annoint Titania's eyes (and will eventually fall in love with Bottom).
In Act 3, he is the only character involved with the conflict of each plot line.
1. He has instigated the duel between Lysander and Demetrius.
2. He has changed Bottom's head and run away the other actors.
3. He arranges Titania to fall in love with Bottom.
He is then involved in the falling action of each plot:
1. He fixes the four lovers.
2. He fixes Bottom.
3. He fixes Titania after Oberon is able to steal the changling boy.
And, of course, he brings about our final resolution of the play by apologizing for the play if you were offended.