Ok Here we go.

Sir Spike's Quest.

Theme/Plot:

So we enter the story with Sir Spike and the Cutie Mark Crusaders in the midst of an imaginary adventure to defeat Rarity, Queen of the Night.

Ok, let me just say that I love this idea. It's not exactly original, but it is an extremely light and fun premise to build upon if done right. Perfect one-shot material.

Execution:

Alright, here is where we run into some of the problems.

First things first, this story is written in the present tense, which is rather difficult to do if you are unused to it. There are several instances in this story where the writer lapses into past tense and these need to be corrected.

There are several instances of words and phrases being repeated to often and to close together.
For example:

Rarity, the queen of the night sent out a letter to all of the knights in the realm:
“If anypony can defeat me, I will resign as queen of the night and they shall have my hand in marriage.

This repetition is unnecessary and it serves as an eye sore to the reader.

The paragraph structure needs some work as many of them are far to short. Stick to the one idea per paragraph rule. If your still expanding on an idea, you dont need to start a new paragraph.
For Example:

Finding myself at a loss of what to do, I summon up the courage to inch my head out and look at what monstrosity Rarity had unleashed upon me.

I glance around the brightly lit lair and lock eyes with a beast so scary, a beast so gruesome, a beast so fluffy that Fluttershy would be conflicted whether to hug or to run from it.

The idea of Spike looking for the beast is not interrupted, so these two paragraphs can be combined.

The dialogue needs a good going over, as there are some problems.
For example:

I could just about make out some laughter from one, no two... THREE ponies!?

“One of them must be Rarity... who are the other three?” I say out loud to myself.

It looks like the story wasn't given a proper going over before getting published. I suggest the writer goes through it a few times to get rid of these errors.

I strongly feel that this story could benefit from some visual breaks whenever the narrative leaves the imaginary and enters reality. This can be accomplished by adding a few more blank lines between paragraphs during these transitions. This will make it more obvious to the reader whenever the perspective changes. The part where Spike gets knocked out and wakes up on the bed could also use a visual break.

All in all, there are a lot of structural issues with this story that are holding it back.

Characterization:

I thought the way the characters are portrayed as the perspective switches between the imaginary and reality was really well done for the most part. Also the dialogue was nice and campy. All in all, it was very light and fun.

The SpikyBelle shipping here was open-faced and enjoyable for the most part. I will say that the other ships you eluded to are probably unnecessary. Unless you are going to expand on these relationships or use them for plot or character development, they just serve to dilute the story.

Grammar:

Now this is not my area of expertise, but it is very obvious that this story needs to be gone over with a fine tooth comb to eliminate all the grammatical errors present. There are a plethora of missing commas, apostrophes, and improperly capitalized words.

Closing Thoughts:

This story has a lot of potential to be a very carefree and fun read. It is, however, bogged down my a multitude of structural and grammatical issues. I highly suggest getting a proofreader to give it a go over, as they will help you identify most of your issues and give recommendation on how to correct them.

All in all, this is a story that needs a lot of work, but after its fixed up it will be fun and enjoyable story. Definitely worth the writers time to address the flaws.

PS. I absolutely love the line:

jump back as a fabulous bolt of energy slams into the ground