Piccolo would be proud.

Sexy Nightmare.

@FanOfMostEverything  
Eyup, I thought of the same story.
 
But I’m more inclined to think ‘Fight instinct said ‘block/deflect’, not ‘run away’’ would be the reason, so a simple tactical mistake against an unusually strong attack.  
Heck, even magic blasts seem to favor defense, in the sense that two mostly-equal opponents can consistently block each others’ attacks (so, since both can successfully defend but neither can successfully attack, it’s a clear bias towards defense)  
If someone jumps around swinging normal, but quite strong, punches and you manage to deflect or outright block all of them, I wouldn’t blame you for reflexively trying to block it even when his fists glow and he launches an unblockable ‘chi punch’.

I can’t help but think of a certain story…

@Background Pony #A9D8  
I would actually love to see that.
 
Then 1000 years later NMM returns and banishes daybreaker then daybreaker her again…
 
A neverending cycle dooming both to repeat it over and over again…

And in a thousand years…
 
|| Daybreaker comes and kicks NMM’s butt. ||

@Ciaran
 
That was basically my headcanon. Nightmare Moon was so full of rage (and arrogance) she thought she could take on the Rainbow Beam of Deus Ex Machina.
 
Either that, or part of Luna wanted Celestia to stop her, so she was holding back.

@Evowizard25  
People do stupid things when they’re mad, and have just successfully overwhelmed the ego of their host body and manifested themselves upon the meat body world of mortal ponies. Happens to everyone.
 
_{or so I’ve heard}

Calm down guys, you knew this was going to be a NMM wankfest from the get go so just sit back and relax. AU’s don’t have to make too much sense, hell in the show Celestia had to have beaten NMM without the elements more often than not for those AU’s to happen but there really is no indicator she is able to.

@Veylon
 
Power nullification aside, what about the second time they were confronted by the Elements before they were hit? They both knew how dangerous they were at that point, and Nightmare was even briefly intimidated by the “objectively useless shiny rocks” they had taken the form of then before they transformed back into their true forms.

@Erosthanatos  
I guess you either have to decide that everyone - hero and villain alike - is an idiot for worrying about the objectively useless shiny rocks or else there’s something about the Elements that makes dodging impossible.

Finally, someone acknoledges it. Neither even tries to teleport.
 
@Evowizard25
 
Edit: But then again, you’d think the Elements might even have secondary powers at this point, like preemptively depriving the target of any means of resistance or escape.

@Evowizard25  
“Teleporting… is useless.”

You know, I honestly think Nightmare couldn’t teleport. She isn’t dumb and wouldn’t have dared take the Elements head on. Given how they’ve worked before, it honestly feels like they sorta negate the target’s attack and locks on to them. I mean, Discord and Nightmare were defeated by them twice by the elements hitting them. I don’t think they’d be fools enough to take them head on twice. If they could teleport, they would have.

“The beam is enormous! There is no escaping!”