Author's Note
Stoic Duty and Firm Stance were just two names I came up with for the guards. Any relation to pre-existing OCs is irrelevant. (You try coming up with guard-sounding names that haven't been used already!! 
)
The Sisters of Two
"Get him off me!! Get him off me!!" Celestia yelled to her sister as repeated grenade fire impacted around her.
"I'm trying, but it's not working! My shots keep bouncing off his helmet!" Luna yelled back, aiming down her sights at the rear of the head of the grenade-launcher armed pony .
"Don't shoot him in the head! Shoot him in his ammo pouch!"
"In his what?"
"The bag over his butt!!"
"Heh, you said butt. Okay, it's not working....Oh, shit he turned this way!! Ow, ow, ow! Get him! Get him!!"
"I can't afford another hit right now, just avoid him for like ten seconds!"
"I won't last ten seconds!"
"Okay I'll shoot him but, since I have the aggro guns, he going to immediately come for me. As soon as he turns empty your gun into his butt-bag!"
"And if he rounds on me?"
"Then we rinse and repeat, make sure you reload quick though!"
"Yikes!! That was close. Okay, ready!!"
"Firing!!"
"...."
"...."
"Whew, glad that's over." Celestia wearily chuckles as their opponent dies in a fire-filled explosion.
"Yeah!" Luna grinned before spotting movement. "Tia, behind you!!"
Bellowing in anger Celestia spun around and eliminated the attacking group of five enemies with five well placed head-shots. "Dammit, I'm trying to heal here!!"
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Heavy panting and muffled curses could be heard coming from the Princesses' play room. The guards outside nervously trying to keep from snickering or barging in dependent on the sounds they heard. Only having been berated for previous actions kept Stoic Duty and Firm Stance from acting on their impulses.
"Who gave them that game again?" Stoic asked his partner.
"Some kid named Button Mash from Ponyville gave it to them for the Summer Sun Celebration." Firm answered, a quiver to his tone the only indicator of his mirth as another string of curses to wafts through the door.
A grin trying to slip onto his face, Stoic mutters "Luna sure knows some archaic ones, but who knew Princess Celestia was so well versed."
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Meanwhile in Ponyville...
"Okay Rarity, I'll go up this side. Then you go up that side till I tell you, but wait....." A series of screams and gunfire interupted Sweetie Bell before the Mission Failed screen popped up.
"RARITY!!" squeaked Sweetie Bell "What happened?"
"Honestly, Sweetie Bell!" Rarity chided her little sister. "You don't have to yell, I'm sitting right here you know."
Sweetie Bell face-hoofed and sighed, "Just tell me what happened."
"I don't know darling, I went around that corner like you told me to. But, then I saw one of those goons and I shot him and he ran around the corner. Then something down the hallway at the end shot me, and I died before I could get to cover."
"You were supposed to wait for my signal before moving up, sis! I was trying to tell you I had to distract the machine gun, so you could shoot the pony firing it."
Pushing her controller away Rarity scoffed, "I don't see why we even need to play this silly game. It's positively barbaric."
"Aww, c'mon sis! You said I got to pick what we could do tonight." Sweetie Bell pleaded with Rarity, "Besides, Applejack and Applebloom already beat all three parts; and we're not even done with one. Anyways, Applebloom challenged me and you to a match for the next Sisterhoof Social."
That little spark of competition that Rarity had with Applejack over anything sisterly suddenly flared. "Alright, we'll try again. And I will try to listen to you instructions more carefully." Rarity flashed her sister a smile before picking up her controller again and settling herself.
Sweetie Bell beamed with pride. She knew it had taken the Apple sisters a month to finish all three parts, but she was sure she could push Rarity to try and finish them in two weeks. Remembering that Scootaloo and Rainbow Dash had spent three days of straight playing caused a small chuckle from Sweetie Bell. I'm not THAT crazy. "Okay so this is what we're going to do, sis! ...."
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Meanwhile...
"Okay! I've got the button layout card set up, check. I've got the guidebook earmarked for the various maps, check. I've got my favorite sister-in-law on my team, check!" A certain purple alicorn princess fidgeted a little, making sure everything was just so. "I've also got a breakdown of what weapons and upgrades we should buy and when. And I even picked up some of those minotaur Red Bull energy drinks!!"
"Twilight? Why exactly are we doing this again?" Cadence yawned on her end of the headset. The sun had already set a couple of hours ago.
"I told you, we need to beat Princess Celestia and Princess Luna!"
"But why do we need to beat them again?"
"Because Princess Luna challenged me and you! We're supposed to face them in a best of seventy-one, team versus team match in a month."
"You just want to beat Celestia, don't you." Cadence deadpanned.
Raising her muzzle slightly and closing her eyes in a mock of the Canterlot style, Twilight answered, "Yes. Yes, I do." Lowering her head, her eyes flashing in determination, "Now finish your character selection screen so we can start the first mission!"
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Meanwhile...
Button Mash stood in his bedroom staring at the night sky, his parents already asleep and his father's fake mustache adorning his muzzle. A wicked grin slowly spreads across his face, "Soon the downfall of Equestria shall begin!! Mwa ha ha ha!!"
His maniacal laugh is cut short as a pink pony pops out from behind his dresser, "Oh no you don't! I've already taken plans to stop your little scheme!" Reaching out with snake-like speed she takes the mustache from his muzzle and attaches it her own. "Remember, I'm watching you." She threatened, before slipping out the door.
"Argh..{the rest of this line is deleted to protect readers and because a colt as young as Button Dash should not know such words, Yours
Truly: The Pinkie Pie Censorship Bureau}