When the Masky Ponies have Twilight, Spike, Spy, and FluttershyBrainwashed and Crazy, and they're overcoming the others, Medic Ubercharges Applejack. She temporarily turns into an alicorn, and unleashes a World-Healing Wave of Honesty that heals everyone and merges Heavy and Fluttershy into FlutterHeavy.
Mayor Mare standing up to Nightmare Doctor. He's an alicorn with armor based off of Nightmare Moon's and has been effortlessly defeating the element bearers and BLUs that have challenged him. She has a lead pipe. It's a Curb-Stomp Battle, of course. But the fact that she didn't flee when she could speaks volumes about her courage.
Mayor Mare: "I'm tired of my little ponies being harassed by monsters and madmares. Time to start doing something about it. BRING IT."
When the heroes are losing to Nightmare Doctor, Barricade (who's bleeding internally) says goodbye to her daughter, then charges the Nightmare with Soldier. They knew they had no chance and attacked anyway. When he's preparing to Mind Rape her, she spits in his face and yells "Celestia Regina!" It's an expression of loyalty to the throne.
Trixie VS The Flim Flam brothers. Flim and Flam show up exploiting an absurdly obscure law that allows them to lay claim to Sweet Apple Acres. Trixie, without even throwing so much of a spark of magic, invokes Screw The Rules, My Friends Have Connections to basically browbeat Flim and Flam into giving up and leaving. Flawless victory.
Whenever a battle involves one of the fused forms facing down a Nightmare, it will have at least one moment like this.
FlutterHeavy uses her weapon to pummel Masky Earth Pony with bullets, forcing it to retreat.
AppleDemo blasts away at Nightmare Grave while stickybomb jumping over the rooftops.
EngiRarity keeps Nightmare Doctor on the defensive with some expert use of turrets.
Scoutie Pie, with help of another fusion, zips right past a barrage of laser blast and takes down the powerful Nightmare Harmonic.
RED Medic, using Sasha, stops an avatar of Slenderman from reaching RED Heavy by somehow tapping into the magic that is empowering both their BLU counterparts.
Queen Chrysalis is now a good guy. Heck. Flipping. Yes.
Discord refusing to work with Slendermane. And would "rather become a Bastion of Order" than let him have his way.
Slendermane's been abducting foals across Ponyville. When the heroes arrive at Berry Punch's winery, they find Berry Punch riding Slendermane, stabbing it in the face with a broken bottle. It's like taking on an elephant with a paperclip, but regardless, Berry Punch took on an Eldritch Abomination for her daughter.
Berry Punch: "YOU LEAVE MY PINCHY ALONE!!"
Cheerilee getting herself out of Nightmare Gardener through sheer force of will (and help from the Doctor, Rainbow Sniper, etc.)
The Changelings' Big Damn Heroes moment, especially with Chrysalis pretending to be Slendermane.
The REDs are missing 3 team members. The remaining members and Merasmus Hold the Line against an army of robots, at one point destroying fifteen out of twenty giant scouts and two out of nine tanks, all of which were attacking at once, with Major Bomber and Sergeant Crits, two of Grey Mann's toughest robots.
The second time Sniper takes on Christain Brutal Sniper, he kicks him out of his head and takes control of his Nightmare - enhanced body.
Merasmus: "I AM MERASMUS. I AM A WIZARD OF TREMENDOUS POWER. AND FOR ONCE IN MY GODDAM LIFE, I'M GOING TO GET THE RESPECT I DESERVE!!!"
His attack then not only staggers Vagineer, but his BLU counterpart, Nightmare Clockwork, through their link, and through him, all the Nightmares attacking Ponyville.
Trixie transferring BLU and RED Engineers memories of imprisonment (on the distant planet and the Australium mine, respectively) into herself, thus not only relieving them of a collective 3000 years of pain and loneliness, but also having to shoulder them herself! Even better, she nearly goes Nightmare from the pain of it, and fights it off.
In Chapter 5, BLU Sniper and BLU Scout getting Brickhouse, a rich Jerk Jock pegasus, fired from the weather factory by making Cloudsdale think Brickhouse is taking advantage of Rainbow Dash's blindness. To top it off, Snowstorm, Dash's boss, bought the factory from Brickhouse's family, stripping him of influence among the employees.