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 chuck 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.
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 BSOD 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*