Littlepip kills the first alicorn they meet by dropping a train car on her.
How about the way Littlepip eliminates Mr. Topaz, a dragon. With Calamity flying her towards the massive thing, she drops a bunch of grenades down Topaz's gullet, the resulting explosion blasting open a wound in Topaz's side that had been made by Calamity
To expand on the awesomeness of Calamity the end of Chapter 18 when he detonates a parking lot of delivery carts to kill pursuing alicorns and ends up leveling three blocks of city.
This troper would also like to nominate the scene in chapter 20 where, after realizing all the mistakes she's made so far and what she's put her friends through, LittlePip decides to empty all of her Party Time Mint-als into the toilet and flush them away, not giving in even when she hesitates on the last tin of the things. Homage showing up to comfort her afterwards is a Crowning Moment of Heartwarming.
In Chapter 28, not only does Blackwing and her Talons help save Stable 2, but when Littlepip reunites with her alcoholic mother and all she gets is a brief glance from her, Velvet Remedy comes up and gives her mum a well deserved smack when she acts like she has suffered.
At the end of Chapter 39, Ditzy Doo pulls a Sonic Rainboom to deal with the Enclave forces ready to obliterate New Appoloosa.
She then does it again, during the battle of Fillydelphia, simultaneously taking out a large chunk of the Enclaves forces, and opening up a hole in the clouds so the Twilight Society can use the Celestia One megaspell to take out the rest.
Chapter 41, the group learns that the Enclave's premier assassination squad, the Wonderbolts are gunning for Littlepip. Calamity forgoes his usual shoot-first policy and devises a plan (or more accurately, a rough idea): convert the Wonderbolts to their side (or at least prevent them from killing Littlepip and the rest of her group), without killing the Wonderbolts themselves. Littlepip remarks that the plan is insane (and inwardly thinks they're doomed), Velvet Remedy more or less agrees, and the entire thing ends up getting increasingly complicated and difficult as more obstacles and threats turn up while they attempt to implement it. One of their companions is killed before they even get started, Xenith is incapacitated before they even get a chance to put it into action (and she's one of the only two member capable of incapacitating a pony without killing them), the entire group suffers multiple injuries, Hellhounds and cockatrices intervene and throw a wrench into the gears- and thanks to perseverance and an incredible amount of luck, it still works. It all depended on Calamity's faith in the better nature of ponies within the Enclave, through sharing his memories of friendship and love over the last two months with Littlepip and Velvet Remedy- and by the time it's all over, all but one of the Wonderbolts stand down.
Velvet completely flipping her shit as the group is rescuing a group of foals from some raiders, not only because they set up their base in Fluttershy's cottage, but because they're making the little ones fight to the death for their amusement, promising the survivor that they'll get their parents' bodies back. Velvet then slaughters all of them with a riot shotgun.
Velvet Remedy: "I've never before in my life hurt another pony," BLAM "far as I'm concerned, I still haven't."
YMMV in this case. For some, Velvet temporarily throwing away her principles and descending into a hysterical Heroic B.S.O.D. afterwards was more tragic than awesome.
DJ-Pon3/Homage gets one in Chapter 40 when she reveals that his current broadcast from Shattered Hoof Ridge isn't live, and rigs an entire power supply into a bomb set to go off right when a group of Enclave soldiers barge in.
DJ-Pon3: "And I have a message for the black armored soldiers who just burst into the Shattered Hoof Ridge Tower: that thing you're looking at with the glowing blue light? A little homebrewed surprise rigged to the spark battery from a weapon made by the motherfuckin' stars! Farewell, you- *cue explosion*
Velvet Remedy gets an awesome one when she and her team of alicorns take over the Enclave's Radio Override System without firing a single shot or causing a single death. Made all the more awesome in that her backup is the result of a single, genuine act of kindness, compassion and mercy (a moment that is easy to forget about), and is paying off in spades.
Enclave Soldier: Back away from the controls, lady. Surrender. You're unarmed, and if I see your horn start to glow, we all shoot. Don't make us kill you.
Velvet: [Pleasantly] But boys, I don't need guns or magic. I have something that's much more powerful than those. [Beat] Kindness.
Enclave Soldier: [Laughing] Kindness? What the hell kind of weapon is that?
Cue a Purple Alicorn removing every bit of ammunition and power packs from their weapons and armor in an instant.
Blue Alicorn: [Drops her invisibility and appears from behind] The kind that earns you friends.
If that weren't awesome enough, she and her team spend the remainder of the battle teleporting injured enemy troops from the front lines to her location and setting up an infirmary to heal them in the hopes that they, too, will be swayed and affected by her kindness.