@drumpowa  
I keep bringing it up for a few reasons. For starters, they really haven’t done much with the concept since they introduced it.
 
Second, I speculate that Dark Magic gets more powerful the more negative emotions and characteristics you possess. Similar to Captain Pollution (who is formed by combining Super Radiation, Deforestation, Smog, Toxins, and Hate), there would be Elements of Dissonance, with dark magic being the secret ingredient. Glimmer, by embracing her bitterness, rage and lust for revenge, would be infinitely more powerful by tapping into Dark Magic.
 
Third, since the writers seemed pressed for time, the audience does not need Dark Magic explained to them. Once people saw the black and green glow instead of Glimmer’s normal glow color, they would know that she was delving into some black and foul sorcery. This is similar to Trixie getting the red glow of evil when she wearing the Alicorn Amulet. As a final bonus, unlike Trixie, Glimmer would bear the fruit of her decisions rather than being able to claim that “the amulet made me do it.”

@Pinkbeardedpony  
Sure, but what kind at this late in the game? Sombra’s dark spell? Only two people know of it Celestia and Amazo Twilight. And even then it was only used at S4 and refered to as alicorn magic by Zecora. So yes, but I don’t know how to make people buy any kind of forbidden technique this late in the game with out major revisions.

@soundtea  
If you are looking at this in terms of a life or death struggle, the paralysis spell only needs to hold Glimmer long enough for Twilight to get next to her, which I would estimate is 30 seconds or less. As soon as Twilight gets within a few meters, Spike can leap onto Glimmer’s face and pluck out her eyes.
 
As for your other claim, I can dig it. However, that is not a narrative that I would assume “from the jump,” and so some dialogue to that effect would have been effective and satisfying. Villainous monologues are always fun.
 
@LOVE  
The utility of Twilight’s non-targeting spells is debatable. Her paralysis spell has always been shown to target multiple creatures, but there is no way to know how Twilight determines or modifies the size of the area and whether she could potentially target a single creature by shrinking the area down. Casting the spell in such a way as to not effect the fillies and colts could prove difficult as you suggested. As for the Sympathy (“WantitNeedit”) spell, all Twilight needs for that is one of Spike’s scales, or one of her own feathers.
 
I concede your point that Glimmer was hiding her exhaustion. I should certainly hope she was getting tired considering that she levitating constantly throughout the fight.
 
Your last point is very similar to the one soundtea was making. The idea that Twilight was getting soft and Glimmer was begging, borrowing, and stealing what she needed to increase her magical abilities has a mote of plausibility to it. I still question her ability to improve so much and so fast, and it really would have gone a long way for them to show a flashback of Glimmer undergoing a training montage of some kind. Her power up is something that I would have to see to believe.
 
Anyhow, to all the fans I have been chatting with, I would like to ask you a question. I think showing Glimmer to be using Dark Magic would has increased her menace, fit with the lore, and helped me buy her dramatic improvement in magical potency. Do you agree that her brandishing forbidden magic would have improved her triumphant return?

@Starswirl  
I can get behind that her talent is research (what I like to do) and refinement; taking what’s established and improve on it. Though I’m not sold on using the scroll having inherent power. You can say that Starlight could have used the map as siphon but I feel that requires more of a stretch and concrete evidence than my “she worked hard” explanation and even that still wouldn’t explain “combat experience and willpower”. I still say her willpower was fueled by her revenge lady-hard on.
 

 
@Pinkbeardedpony  
My own hypothesis is that Twilight is rather by-the-book. She goes only by the parameters of what she thinks is possible and never goes outside the box (which is why at times, she’s frustrated with Pinkie Sense earlier.) Starlight can take spells to the next level which is why she seems stunned that Glimmer was doing what she was doing.
 
Also, let’s keep in perspective that this battle isn’t a physical one, it’s still a light show just a lesser CQC-esque light show compared the last bombastic DBZ-esque finale. But the end result is once again an impasse or fighting for all eternity (ala Adventure Time Goliad v. Stormo).
 

 
@MezzoDragon  
At the very least (though I haven’t seen season 6 spoilers) they haven’t re-reworked the logic of magic so it seems the map is permanent. Cutie Mark Magic seems to still be a theme for S6.
 
 
@soundtea  
That’s what happens when you skip leg spell day.

@MezzoDragon  
It’s not the kind of thing that should be abused for plots anyway.

@Starswirl  
But how often does she actually go for combat? Plus what love said about those being big, dumb, easy targets. Unless there’s a crisis at the moment, Twilight is likely usually reading or trying out new spells rather than constantly doing magic workouts.

@Pinkbeardedpony  
You’re welcome, and I also want the writers to explain that. However, it seems like they either lacked the time and/or desire to explain it. This show is aimed at young girls, but having an ordinary (albeit talented) unicorn being this much of a match is almost enough to break the suspension of disbelief.
 
Twilight was even afraid of her for reasons I can’t fathom. It would make sense if they had multiple encounters to determine Starlight’s threat, but no. Aside from “Where’s Waldo?” moments, she never really appeared in any episode between the beginning and ending.
 
It is a waste of her character. Much like how the Cutie Map was a waste of a perfectly usable mcguffin. 26 episodes; only used four times.

@LOVE  
Aye, that’s true. They did show Starlight stalking Twilight, so it’s reasonable she also did so to take her measure and prepare for the fight.

@Starswirl  
She mostly fought monsters, big giant slow monsters that probably are easier to hit with lasers than a small unicorn.

@soundtea  
Here’s the problem with saying Twilight was cushy: She wasn’t.
 
She fought a bugbear on a wedding day, she fought Tirek, she fought a Tatzlwurm, she fought Starlight near Our Town, she used her freeze-spell on small and large groups… Twilight Sparkle and her friends were in action so often, the rest of Ponyville didn’t even panic anymore when the next monster attack came.

@MezzoDragon  
I know that pal. What I wanted is for the writers to have incorporated a reason why Twilight’s getting slapped around that makes some sort of sense. It makes me ornery when they don’t do that. Let me say that I appreciate your civility, most people would tell me to soak my head for all the kvetching I do. :)

@Pinkbeardedpony  
There’s a good reason why she wouldn’t bust out her Paralysis spell: It might accidentally hit Rainbow Dash and disrupt her race. Battle over. Also Starlight was shown to easily be able to get out of the crystal prison spell, and obviously has a really power Telekinesis. The Paralysis spell probably would not have worked as well on her.
 
Want it Need it would have been bad too. It quickly got out of control when she used it before, why would she expect to somehow control it and not get a bunch of the fillies to start fighting over whatever object she casted it on, disrupting the race, and thus losing the fight? Also there wasn’t exactly any tangible objects in the Flight Camp that the spell would have worked on. Everything was made out of cloud.
 
And while Twilight shows her frustration in the battle more, at the end Starlight looks just as exhausted as Twilight. She probably is just hiding it because she wants to feel superior and make Twilight suffer.
 
Last but not least, Starlight’s power up. Why would she just ‘observe’ without preparing? That doesn’t make sense, of course she was preparing. I also have a good reason why she got stronger vs when she fought her in the village:
 
Starlight was out of Practice in Our Town.
 
She was in the middle of bumble fuck nowhere, so she had almost no access to any spell books or other form of research. Additionally, even if she did she couldn’t really practice at all because she was hiding her cutiemark from her village. So she probably almost never used her magic whatsoever. But when Starlight escaped, she was shown to be able to just walk around Canterlot freely. She also had access somehow to Starswirl’s archive and any other spells in Canterlot. So she now had the time, ability, and resources to train her magic further. And Twilight doesn’t exactly sit around all day looking up new ways to horribly maim and kill ponies, she learns all sorts of spells. Starlight on the other hand, was specifically preparing for her confrontation so she exclusively studied combat spells. Hence why she was a better fighter.
 
This happens with athletes too after injuries or spending too much time on their asses. They’re able to really bounce back super fast, and sometime even get better than they used to be.

@Pinkbeardedpony  
There’s no sure bet it would work. Starlight busted out of the crystal thing faster than Twilight could. And the hold thing doesn’t elaborate if it shuts off using magic on your end as well.
 
Remember that Twilight was crazy strong in pre-MMC, and there’s absolutely nothing stating that only one magic prodigy can exist. Starlight likely just never seriously explored that part until she started stalking Twilight out of revenge.
 
In essence, she level grinded the fuck out of her arsenal while Twilight was generally cushy.

@Pinkbeardedpony  
Twilight is only as strong as the plot requires.

@LOVE  
Which begs the question, why does Twilight rely on spells that require you to aim at and hit your target (when she can’t hit the broad side of a barn)? That is why I suggested earlier that she bust out her paralysis spell (first used in the Castlemania episode) or the Sympathy spell (AKA Want-it-Need-it, only use was in Lesson Zero).
 
The other issue here may be that Twilight “sells” her battles more than Glimmer. If you are not familiar with wrestling terminology, selling means that a combatant exaggerates any hits they take and wears their discomfort on their face to a greater degree than their opponent. Twilight makes no attempt to hide that she is struggling during the battle. I presume that she is not putting on a show to entertain the pegasus fillies and colts. ;)
 
Lastly, she was clearly outmatched against Twilight the second time they fought (as evidenced by Glimmer fleeing to avoid capture). Her fighting Twilight so much more effectively is a bit jarring, especially considering that we only saw her spying on Twilight (which makes her cunning, but not stronger in a direct sense).

@Pinkbeardedpony  
Starlight really didn’t seem to be more powerful than Twilight. She was just much more prepared and combat savvy.
 
It doesn’t matter if Twilight can turn Starlight’s insides into molten lava or something, pretty much all spells require you to hit your target with a beam of somekind and Twilight couldn’t aim worth shit, and Starlight was able to Dodge all her telegraphed attacks. That’s why she was able to match up to Twilight, because Twilight has no real combat training.
 
I think this shows that Starlight specifically prepared for this moment over the season. She wouldn’t have activated the time spell unless she was confident she could take on Twilight anyway.

@Starswirl  
I like your proposals. The spell was crafted by Starswirl the Bearded was it not? Furthermore, the map was created by the primal forces of the planet itself. I can mind caulk together the narrative of Glimmer leaching off either of those artifacts for a power boost. Smart thinking. /)

@Pinkbeardedpony  
Yes, that was my point.
 
As for possible explanations, maybe her special talent is researching and refining magic for particular circumstances.
 
Another possibility is that she siphoned magic from the scroll and/or the map while Twilight and she fought. As in, as long as they were stuck in this battle loop, Starlight had another reservoir of power to draw upon. Neither of them could cast the spell without these items, after all.

@Starswirl  
It sounds to me like you are saying that the story would not have changed in a meaningful way whether Twilight squashed Glimmer or not. I agree with you if that’s your point. The discussion that has been going on recently is how it was possible for Glimmer to improve her abilities so fast and to such a preposterous degree. I have yet to hear a persuasive argument for how she could have pulled it off. She needed a secret weapon, and I say it was a deal with the Devil, I mean Discord.

@Pinkbeardedpony  
Well, I think it’s possible Twilight could overpower Starlight at some point. Starlight was very prepared for this fight, but we also saw Twilight’s skill shine when she copied Starlight’s crystal prison spell.
 
Thing is, even if she (Twilight) could’ve won by force, she would’ve needed to do so in a way that instantly shuts off Starlight’s ability to affect her surroundings while also being so subtle, the ponies around them don’t notice. And that’s pretty much impossible.

@drumpowa  
Your theory is interesting, but has odd implications. It sounds like you are saying that Glimmer was able to train so hard that she caught up to Twilight. First, how much time could she have had to make this amazing transformation? I’m thinking that she had half a year at most. Here’s the rub. I will give her a nod in terms of learning and creating spells quickly. I would concede that she is Twilight’s equal in that regard. However, you would require much more concrete evidence to convince me that she was able to increase her combat experience and willpower to match Twilight’s in such a short span of time. Nor am I willing to concede that Glimmer is somehow able to overcome Twilight’s pedigree (daughter of a major character from an older generation) or her one-of-a-kind education (Sunset Shimmer is not canon), being the former student of Princess Celestia.
 
Let us not forget that the thesis of the show is that true ultimate power comes from living a live of friendship, rectitude and love. Twilight solving friendship problems and forming loving bonds with other ponies should ensure that she outclasses Glimmer now and in the future. Glimmer seemed to be a creature of hatred, rage, and spite. Unless she was rocking some sweet dark magic, I don’t understand how she could overtake Twilight anytime in the current millennium.
 
Pulling a Batman / Dr. Doom might be a good explanation for Glimmer’s upset, but there’s a problem with that as well. This type of explanation requires that Glimmer exploit some inherent weakness of Twilight’s to keep up with her. Unless you are claiming that Twilight having bad aim is a real character trait (she doesn’t wear glasses even though she spends a huge amount of time reading), then I didn’t see any story element of that sort present in the finale. The duel was knockdown, drag-out fight. Glimmer didn’t take advantage of terrain, she didn’t take any hostages, she didn’t rake the eyes or kick Twilight in the groin. It was a test of magical power and aerial maneuverability, nothing more.

@Pinkbeardedpony  
I don’t know if you know computers, but if I were to compare SG and TS like they were computers, Twilight has High HD Storage space but low on RAM. Starlight having much higher RAM but kept upgrading her HD Storage. So while TS is capable of a multitude of tricks they tend to be raw or unrefined. One instance is Scare Master where she did the freeze spell she was fine with 6 ponies but in Hoofields/McColts episode she took on a BUNCH and while she did it, she was clearly struggling, the mark of someone who doesn’t refine their work enough to make it earlier on them.

@Pinkbeardedpony  
I guess Celestia taught her too well.

@MezzoDragon  
Your argument is perplexing to me. All of the spells I listed have been cast, by Twilight, in the show. I am not suggesting that she turn Glimmer into a goldfish and watch her asphyxiate to death while smirking and lighting a cigar. Heck, I wouldn’t care if Glimmer “no sold” every single non-damaging spell Twilight threw at her. The point is that Twilight reaches deep and throws everything she’s got at Glimmer and then Glimmer just throws her head back and laughs when nothing sticks. Twilight using nothing but the most basic unicorn magicks when she is the greatest living mage in Equestria is portraying her as coward, a nincompoop, or both.

@Pinkbeardedpony
 
@Drasvin
 
I have said this before but noone seemed to respond to me…maybe she just worked hard to get that powerful. There was nothing in the show that I recall that said that you couldn’t build your power like a muscle, that you were given a static amount at birth. Remember that Twilight when she was as a filly had worked to get to a point where while she had trouble doing magic but built up her power like a dam. It didn’t manifest until the rainboom happened and opened the floodgates to all her built up power and subsequently her power incontinence in the previous seasons. Now it seems her brain has caught up somewhat where her incontinence isn’t a detriment.
 
Starlight Glimmer as quoted by Big Jim is proto-Twilight and remember she had the ability pull a CM off of a ponies body by herself and replace it. If that’s doesn’t require alot of power to pull off I don’t know what does. Secondly, from her backstory had no indication she was a prodigy and her lack of what may be formal education tells me after her declaration, she trained to be able to do what she did in the premier. Fast forward after the premier you see her stalking Twilight. We only see her for two episodes in the background but the fact that she is and at least twice means she could have done it for more than that. So we see she does her research, plus her access to libraries means she can pull out Starswirl’s spells and do what she needed to do.