This is how Twilight and Optimus expose Midnight goes in A Canterlot Wedding.
[We see Twilight's friends, the Autobots, Celestia, "Cadance" and Shining Armor at the receptional]
Shining Armor: [to Twilight] You want to know why my eyes went all [bells jangling]? Nuh! Because ever since I started having to perform my protection spell, I've been getting terrible migraines. Cadance hasn't been casting spells on me. She's been using her magic to heal me! And she decided to replace her bridesmaids because she found out the only reason they wanted to be in the wedding was so that they could meet Canterlot royalty! And if she hasn't been on her best behavior with your [stomps hoof] friends, it's because with me being so busy, she's had to make all the decisions about the wedding! She's been completely stressed out because it's really important to her that our big day be perfect! Something that obviously wasn't important to you! [gasps] Now if you'll excuse me, I have to go and comfort my bride. And you can forget about being my best mare. In fact, if I were you, I wouldn't show up to the wedding at all.
[Before he could comfort his "bride", Optimus shuts the door and locks it]
Shining Armor: Optimus, what are you doing?!
Optimus Prime: Twilight is right, Shining Armor, because I also know the truth. The truth about a plot. A massive deception.
Shining Armor: What plot?
Optimus Prime: Well there's a sinister plot to take over Canterlot. I have proof that the Cadance you see is fake.
Princess Celestia: Show us the evidence.
Optimus Prime: Matrix of Leadership, show us the evidence.
[The Matrix of Leadership shows everyone the evidence that the Cadance they are seeing is really the Predalien Midnight]
Shining Armor: My bride is a monster in disguise?!
[The Matrix of Leadership then shows them Midnight contacting Thunderwing]
Midnight: Master. Shining Armor's love is well within my reach and still, besides that Alicorn and Optimus, no one has a clue I'm not really that pathetic princess of love.
Thunderwing: Good, my apprentice. Soon we will have control over Equestria's capital and Canterlot will soon belong to Unicron.
Midnight: The more I pretend to be Cadance, the more they share my beliefs.
Thunderwing: Good. Once more, Unicron's chaos will reign supreme. Then we shall have Equestria for ourselves once and for all.
[The images disappear]
Shining Armor: I can't believe Midnight and Thunderwing would