@Keith Mowz  
That too is a possibility.  
Few people seem to be considering the possibility that Tirek himself probably isn’t some “empty vessel” that is only as strong as the amount of magic he absorbed, and that the one he absorbed is simply used to recharge his own powers.

@Background Pony #45BD  
Or its a rock-paper-scissors thing, and Discord’s magic used in a discorderly manner is more effective against alicorns than Tirek’s basic fiery magic.

@IceKitsune  
No sure about the whole “more experienced” thing.  
When it comes to battle, we’ve never seen the princesses do anything more than what Twilight did on this battle. During the NMM/Celestia flashback, that was mostly was Luna did and she’s supposed to be “experienced”. When Twilight said she didn’t have experience, I think it was quite obvious she was reffering to holding back the power to accomplish menial tasks, since that was what she had trouble doing during the episode.
 
When the battle started, she didn’t seem to have nay trouble teleporting where she wanted, or firing energy beams with great accuracy.
 
The point is I very much doubt any of the other princesses would’ve done better than Twilight herself. If they could, and Twi only stalemated him because she was “inexperienced”, then why not give all the power to Celestia or Luna to defeat Tirek?
 
The reason, I think, its pretty obvious.  
They knew Tirek was already stronger than even the 4 of them combined (and he had Discord on his side too), so hiding the magic was their only option. That Twilight was able to do so good against Tirek, was Imo, something none of them had predicted. It is a testament to Twilight’s own skill and ability. The other princesses had 0 faith on their own ability to fight Tirek even with all their power combined, showing that what Twilight did was something that most likely only she could do. Twilight already had all the necesary “experience” when the battle started.
 
Anf about the power comparition thing, it is simply too messed up to make a valid powerscaling. We know for a fact that Discord is more powerful than Celestia and Luna together. Much more so in fact. And yet, when Tirek ate Discord’s magic he barely grew anything compared to when he ate the princesses’s power. Heck he grew more when he ate the Mane5’s power (though I guess we could argue that since Spike was there, his dragon blood could’ve played a part on that I dunno).
 
Even if we ignore this inconsistency, the only way this could work would be if:
 
Discord = 3 Alicorns.  
The power of all Unicorns/E.Ponies/Pegasi in Equestria = 1 Alicorn.  
That’s the only way Twilight’s battle (and stalemate) with Tirek would make sense.

I love how, during this entire scene, Twilights Magic has hers, Celestias, Lunas, and Cadences auras together. Truly awesome.

YEAAAAAAAAAAAHHHHHHH!!!!!!!!!!!!!!!

“And then Twilight smote The Tirek, and all was laid to burnination.”

@Background Pony #45BD  
Plus my point with that was mostly Twilight didn’t know how to use her own Alicorn Magic and all 4 of them combined stalemated Tirek who had just about all the pony magic in Equestria plus Discords.
 
If she was more experienced she likely would have beat Tirek rather easily. And that makes the individual Princesses pretty powerful since 4 Alicorns is at least equal to every pony in Equestria and Discord combined.

@Background Pony #45BD  
I don’t think that means that no one else could have done it, its just she was the most likely one to be able because of her talent.
 
Twilight was the least risky. The others may have been able to do it but Twilight was the safest of them because of her talent, and the fact that Tirek didn’t know about her.

@IceKitsune  
“You represent the element of magic, if there’s anypony capable of doing this, is you”.
 
I don’t think its something the other princesses can do. Its very speciality-oriented.  
Celestian & luna can move the sun and moon, butt its canon that a well-fed Chrysalis has more power, and also that Shining Armor can create stronger barriers than them.
 
Imo they implied Twilight was the only one who could effectively use the power of all princesses because that’s exactly what her cutiemark/speciality is. She represents magic.

This whole battle is one reason to sell the toys, epic fight scenes don’t pay for themselves you know.

Looked like this scene was done outside of Flash and more hand animated for effect. Props the DHX and Hasbro, though.

Dragon Ball.

Can we just point out that
 
THAT’S FUCKING AWESOME

@RIG  
Or an opponent with anti-beam coating, or someone in water. Beams are crap in water. She needs a missile or bullet attack.

@IceKitsune  
Well, their powers combined it seems.

Mein Gott…

I really like that the combined power of 4 Alicorns is strong enough to stalemate the combined power of all (or at least almost all) pony magic in Equestria as well as Discords magic.
 
This is also while it was being used by Twilight who said that she wasn’t well experienced with her own Alicorn magic alone.
 
Imagine if one of the other Princesses would have gotten it instead. This pretty much proves that all of the Princesses are really god damn strong.

Twilight Sparkle used Hyper Beam!

!24.media.tumblr.com/8dc3891151ea7472d8b9ed23cb918fe0/tumblr_mjqenzCk9l1r4g1p5o1_500.gif!
 
Seriously, goosebumps everywhere and tears in my eyes while watching this scene

YO, WHEN’S MAREVEL?!

MASENKO-HAAAAAAAAAAA!!!

as beams are her only weapon, she would be in trouble if she finds someone with a reflect spell.

TWILIGHT BEAMU

Shinku-HORNDOUKEN!
 
-Lumino