Okay, maybe I'll have time to give you a full once-over later, but for now, go back and fix this up yourself instead of coming straight to me. Something in the formatting shuffle screwed you HARD, and this is unreadable with all the random-ass punctuation everywhere.
Your writing is also not technically good. Apart from a truly infuriating amount of tense, subject agreement, and other technical errors (PLEASE keep an eye out for those too when you do your editing pass), You have maybe two good bits of description that aren't dry and generic. Let me give an example.
>Twilight pushed Spike away, kicked the attacker to the ground, jumped and kicked her back-hooves into the skull, which broke under the pressure.
You want to write gore? Write some fucking gore! Twilight just crushed a brittle undead skull with her rear hooves, do some show-don't-tell on that shit! Shards of bone, blood soaking into Twilight's fetlocks, the eerily lukewarm gore against her skin, the zombie spasming before falling limp and tumbling to the ground, brains and rotted flesh getting all over the floor Spiked worked so hard on cleaning.
So that's a direction for improvement. Look for places where description would be fun or where it matters, and describe the fuck out of things.
Another thing for when you go back and rewrite this. You do a very poor job of telling us how Twilight and Spike feel about this whole thing. Sure we can assume they're scared or whatever, but As an experiment, and also because it's not like you have another direction that will be lost if you do this, pretend that Twilight is the main main MAIN character and tell the story with an intimate 3rd-person-limited POV that's inside Twilight's head, i.e. "Twilight's heartbeat quickened as she thought about blah blah blah. She glanced over at Spike, saw how he was shivering, and wondered if blah blah blah." You need little things like this in order to make your characters enjoyable.
I won't bother talking characterization or plot or anything because those come after you've gotten your writing in order. Good luck, this part is always hard.
One more thing. Do this in Google Docs so I can use that nifty comment feature. Then take it to Fimfiction when it's ready for prime time, but ONLY then.