A/N: This is a slowly growing collection of one-shots that has been gestating in my mind for a few years now, and being laid up for a week has finally driven me to write the first one. More may well follow. The basic idea is that, no matter how ludicrous they may be, or how 'squiffed' the original series might need to be to fit together (for example, Gundam Wing and Yu-gi-oh? Yes, the world theoretically could work... if the YGO characters wouldn't be 215+x years old and buried, their stories completed) or 'fluffed' the power levels would have to be (Ranma 1/2 vs Dragon Ball Z. Hey, lets combine these two martial arts anime! That'll work! Boom goes the planet).
So, without further ado;
Crossovers that Should Never Work
Fit the First: "They will look up to me as if to God and say 'Save us!'" (Magical Girl Lyrical Nanoha A's and Neon Genesis Evangelion)
Takamachi Nanoha, age fifteen, frowned slightly as she looked down at the city she was currently flying approximately one point five kilometres above. It looked like a normal city, but there was something off about it – beyond the obvious – that was itching at her mind. Some of the buildings looked almost fake. In fact, almost every building that hadn't been recessed into the ground for protection against the most obvious sign that something was wrong looked fairly fake.
The most obvious sign that something was wrong of course was the giant, vaguely humanoid monster walking purposefully through the fortress-city without a single one of the attending static defences even attempting to halt its progress. The sun had set over an hour ago. The Bureau had sent her here because of a minor dimensional dislocation caused by an energy field interacting with a phasic barrier of some kind – such as someone setting off a very large explosive next to some kinds of shield spell – and yet here this barrel-chested monster was destroying nothing, and being struck with nothing.
The creature seemed to have very thin arms and legs with disproportionately large hands, feet and torso. Its face – if it could be called that, looking more like some cultures' beaked death masks – sat on front of its body, looking like it had been pushed up to allow a second, smaller one to peer through.
[A bogey has been found approaching at high speed from below] Raising Heart informed her, and she looked down. From below?
Of course, a lift. She saw the large, purple form of a second giant, this one far more human-like, shoot to the surface and stop abruptly, force throwing its head up from where it had rested on its chest. Under those forces, any living creatures neck would have snapped, so this had to be a robot of some kind.
"Why is it," she wondered softly and slightly despairingly, "that whenever man must fight monster, his answer is to build a bigger man?"
As a soldier – even in the fleet air arm, as it were – Nanoha knew full well the limitations of the human form. "How is that communications tap coming, Lieutenant Ramirez?"
"Just coming online now, ma'am." The voice of the male operator she was assigned to for this mission came over her communications link from the relay she'd dropped just outside in dimensional space. "Give me a few moments to get you something useful. I'll patch you through to active comms."
"Recieved." Nanoha confirmed, and tapped her earring link to switch channels in one ear.
"Just concentrate on walking, Shinji-kun." An older, confident-sounding woman's voice. Someone who enjoyed life and bounced back like a training dummy.
'They're sending someone into battle and he doesn't know how to make his mech walk?' Nanoha wondered, incredulously. Someone – a young boy? - mumbled something in response.
"It's walking!" A different woman's voice, this one sounding more arrogant and self-posessed.
As if the arrogant woman had spoken too soon, the purple machine tripped and fell, only to be picked up by the monster to the tune of more yelling by the bouncy woman.
"He's getting slaughtered out there. That's it. Ramirez, forget the comms for now, get me a reading on that thing's shield." Nanoha cut the comm back over just as Shinji started to scream as the monster broke his mech's arm.
"Got a force screen of some kind up. The purple mech has something similar, but it's inactive. That shield is stronger than one of our containment barriers, ma'am."
"Very well." Nanoha flicked her staff out to one side, turning it so that the power vanes could deploy without incident.
"You... don't sound very concerned, ma'am." Ramirez did.
"I broke a containment barrier stronger than the ones the buruea uses when I was nine and a half years old. Raising Heart."
[Count 9] The device intoned, and power started gathering.
"While I was badly hurt..."
"And had about half..."
"A linker core left to me..."
[Count...] Even Raising Heart paused to savor the moment, 
"Full Power. Starlight Breaker!" She called, and sent forth the pink beam of doom.
Sachiel, the Third Angel, never even had a ghost of a chance. Starlight Breaker was already a top-grade magical attack spell before Nanoha spent months trying to push more power into it through near twenty four hour training. Mostly for her efforts, Nanoha just got blown up. When she finally got it right, she'd increased its range and tracking ability by a grade each and imbued it with a special ability – Total Seal Break. There was, by definition, not a single noncorporeal defence that could stop her attack, and many corporeal ones would be needed.
Fortunately for NERV HQ and the geofront, it wasn't a Divine (bunker) Buster, and so most of its power was aimed at shattering shields and disrupting energy flows within soft targets, like criminals.
Or, in this case, Angels.
That was, of course, ignoring the fact that the Angels require their secondary AT fields to maintain cohesion within themselves, aside from especially thick armour, thus dissolving when the core is broken.
Sachiel did not have especially thick armour.
Were LCL not so viscous, Suzuhara Mariko would have learned first-hand what it was like to sit in an Eva cockpit that night. Fortunately for her, she had enough time to get to the third floor of the (mostly fake) building she was cowering in with her mother.
Note: Nanoha's phrase "Full Power" in Japanese is "Zenryoku Zenkai", which is a homonym for "Absolute Destruction."