Luna was given the roll of mobile gun platform and provided with a weapon-rig made from lightweight, extra-dimensional ironwood by the EXEC.
She carried the gun—a heavy perfume repeater—on her back, and pushed an angled shield on wheels that extended like wings to either side, with an indentation in the middle that allowed her to see and aim ahead while still protecting her chest. To compensate for the indent, she was also given a durable helmet to protect her head, and a pair of goggles to keep the dust out of her eyes.
The shield apparatus was attached to her rig's harness, and provided cover for her crew: an anthro-pony named Stokes and and an anthro-griffon named Wind Sock.
Anthro-griffons were typically assigned to flight-crews or recruited to serve as paratroopers, but Wind Sock had injured her wing shortly after arriving on the Equestrian front and was reassigned. The idea was that her understanding of wind-conditions would give her some advantage as a spotter for long-range artillery, but in practice she was no better than a trained pony-anthro.
Stokes was an earth-pony anthro; placed in charge of keeping the weapon, and the princess, from overheating. In order to do that she had to stuff cooling canisters into the princess's anus, which had been mechanically augmented using imported magi-tech to accept the canisters and disseminate the cold-magic from the interred windigo-crystals throughout her body and rig. When one canister was exhausted it was exchanged for a new one to allow the twin-barreled cannon to continue firing.
The actual weapon drew power from Luna's pussy, transferred via cable from a device implanted deep within her womb that collected magical power from her body.
The cable had to be held aside when inserting or ejecting a canister, as did Luna's tail. Luckily, the pony princesses, with the partial exception of Twilight, were rather tall, and Luna's ethereal tail wasn't at the mercy of gravity. When it was time to reload, the princess could simply raise her tail above Stokes's head to allow the gunner access to her 'rear ports.'
"What's that?" asked Wind Sock, watching as a figure emerged from a cloud of dust and vapor rising out of the ground where a changeling liebegranate had landed. The figure—an anthro scout—had leaped over the crater left behind by the shell, wearing nothing but a gas-mask and the tags around her neck.
Before anyone could say anything the scout slid into cover behind the shield, scrambling to her hooves and removing her mask. She remained in a crouch, her tags dangling in front of her flat chest.
"They have a mortar!" she cried between breaths. "We lost a lot of fighters thinking we'd outflanked them. It's some kind of gas. Breathing it in makes you..." the scout turned red, then shook her head of short, white hair. "Anyway, we need it taken out."
"Show me where." said Luna, looking at the newcomer out of the corner of her eye.
The scout nodded, then scrambled under Luna, popping up next to Wind Sock.
"See that grain silo? The one fallen over on its side? I think it's behind that." said the scout, gesturing by extending her arm toward the collapsed structure.
"You think?" asked Wind Sock, uncertain.
"That's right." said the scout, a little indignant. "Look, it's the best I can do. I'm telling you, it's there. It has to be. I saw a couple of drones shooting over the top before the gas hit."
Wind Sock nodded her head. "Adjust azimuth, 5 degrees." she said.
“Do you have a radio?” asked the scout. “I lost mine. We need to tell EXEC about the gas.”
Wind Sock handed over her radio to the scout.
"Azimuth, 5 degrees." confirmed Luna, maneuvering the rig to point a little to the right, until the gun was aimed directly at the fallen silo.
Closing her eyes, Luna tensed her muscles. The cannon began to charge, the rig siphoning power from her pussy. Her mane and tail flared as the lights on the cannons lit up along the barrels, creeping forward as they approached full-charge. Her pussy winked and started to drip, and her teats swelled up and began to leak milk.
She opened her eyes, which were glowing with arcane light, and with a sound like ripping fabric the repeater released a hail of perfume-bolts at the silo, battering the metal and pulverizing the dirt. When the barrage was over, a group of doppelsöldners swarmed the position.
"Re... load." gasped Luna, her chest heaving and her flanks slick with sweat. The barrels of the gun were red-hot, and there was steam rising from her ass.
Stokes slapped Luna's flank to trigger the mechanism to eject the depleted canister, which pried open her asshole from within as it slipped out.
It clattered to the ground, the crystal having melted away entirely. Working fast, Stokes opened the lid of the munitions crate and fetched a fresh canister.
"Hurry. Please." said Luna, panting like a dog.
The canister had a bullet-shaped nose that helped with penetration. Stokes lined it up with Luna's asshole and pressed gently at first until the ring opened to accept the tip, then rammed it in using all the strength of her arm and shoulder and making Luna moan at the sensation of having her asshole pried open by such an enormous device.
Stokes was relieved when she heard the clicking sound which meant the canister had been accepted. With a hiss, the cooling process began and Luna practically fell unconscious from the sheer relief, which was so pleasurable that it was almost worth the pain of overheating.
She stuck her tongue out and exhaled a hot breath, her eyes rolling back in her head. At the same time the guns returned to working order, returning to minimum charge almost immediately with an influx of magic born from the overall good-feelings the princess was experiencing.
"I think we got it!" said the scout.
"You think?" teased Wind Sock.
Author's Note
Weird concept, weird story, but entertaining to write. 
