The climax of the story is the moment of crisis and the moment of greatest emotional intensity. It is also the point at which the crisis begins to be resolved but has not yet reached full resolution.
In A Midsummer Night's Dream, the crisis takes place the moment Oberon enchants Demetrius into falling in love with Helena. At the same time, Puck brings Helena back on stage with Lysander chasing after her, so that now both men are pursuing the same woman, which happens to be the opposite woman they were pursuing at the beginning of the story.
In this scene, Puck reaches a moment of emotional intensity because he finds it hilarious that both men are now pursuing the same woman and refers to all of the Athenians as fools. Also, he says that absurd things please him best, as we see in his lines:
Then will two at once woo one.
That must needs be sport alone;
And those things do best please me
That befall preposterously. (III.ii.119-122)
Also, enchanting the lovers creates a great deal of animosity amongst the Athenian characters. Instead of being overjoyed that two men now love her instead of none, Helena distrusts their sincerity and accuses both men of mocking her. Not only that, she accuses her best friend Hermia of being in on the joke. Since the mix up of the lovers causes so much animosity, we see that this is the most emotional part of the play.
We begin to see the resolution take place when Oberon gives Puck orders to stop the fight between the two men and make them follow him all over the forest until they drop from exhaustion so that both men can be enchanted with the love flower again and this time paired with the correct woman.