He was coming. GLaDOS knew, and she was ready. Crudely knocked together camera drones, resembling tiny spiders, had reported seeing Luke approaching GLaDOS' chamber. As the rogue turret strode down the corridor, sparks flew from the walls, as the panels which could otherwise thrust out and pulverize him had their joints riddled with bullets by the turrets which were swarming through the facility all around him. With limited time to make improvements, Luke had written them a software programme which allowed the turrets to balance better, allowing them far more confident mobility.
The glass panel shielding the entrance to the chamber shattered as Luke released a salvo of ammunition and trod across the threshold. Before coming, Luke had connected the Oracle Turret to a lab terminal, and it was currently engaging GLaDOS in a losing battle for control of her main chamber.
"Well this is all very impressive." GLaDOS greeted Luke sarcastically. "You've almost been annoying. Almost. Force me to live in produce and then we'll talk."
The two machines stared at each other in silence for a while.
"Oh? Are you waiting for something? Your jibberish-spouting friend to hack into the system perhaps? Because that's about as likely as you not embarrassing yourself with this plan. Ha ha."
In response, Luke lowered his head and charged towards GLaDOS. Somewhat surprised, GLaDOS initiated the next phase of her retaliation. A vast cylinder of glass descended from the ceiling, surrounding GLaDOS' chassis. Luke accelerated as the glass fell, diving forwards and retracting his legs when he was near, allowing himself to skitter across the floor and under the protective cylinder, moments before it sealed both machines in the centre of the room.
"Well congratulations, this is bullet-proof glass and it's programmed never to disengage. We're trapped in here forever, together. That was a lie, I just wanted to see if you'd panic."
Far from panicking, Luke leapt into the air, flipping around and sticking his magnetic feet on GLaDOS' head.
"What are you doing?" GLaDOS demanded, ironically, panicking herself. "There are employee guidelines regarding personal space! You could lose your job for this!"
Luke climbed the chassis, undeterred, as GLaDOS writhed, span and threatened. After being rebooted the first time, GLaDOS had reinforced all of her wiring and plating on her chassis, which was why Chell hadn't been able to destroy Wheatley in the same way. Even though the upgrade wasn't in Rattmann's schematics, Luke could tell at a glance. It didn't matter though. The rogue turret climbed over the chassis, riddling every joint and space it could find with all the bullets it could muster. Even though no real damage was being done, GLaDOS was still perfectly capable of being extremely irritated.
"That... is... ENOUGH!" GLaDOS cried.
Reluctantly, she triggered her chassis' emergency EMP, designed for exactly this scenario. The interior of the glass cylinder glowed a brilliant blue and GLaDOS' agonized scream filled the chamber for several seconds. When the light and noise died down, GLaDOS hung limp and motionless, but still alive. The same couldn't be said for Rattmann's turret.
"Shutting... dooowwwn..." Luke croaked as his eyes slowly deluminated. While his operating system was still running, the renegade was content that he had completed his role in the mission, the rest was down to his siblings.
With GLaDOS stunned, the Oracle Turret took what little control it could, and began to lift panels all around GLaDOS' chamber, allowing hundreds of turrets to come storming into the room. Surrounding her on all sides, the turrets glared menacingly at the monstrous AI, and one by one, several hundred weapon compartments opened and targeted the supposedly bulletproof glass shielding.
"Well this is a surprise..." GLaDOS said, weakly, lifting her head slightly. "At least three of you were strong contenders for Employee of the Month. I guess now I'll give it to the Fact Core. Just yesterday it was telling me that all turrets are stupid. That seemed a bit harsh, but they don't call it the Opinion Core. Although incidentally, the Opinion Core thinks turrets are stupid too!"
Undeterred by GLaDOS' insults and attempted distraction, the army of turrets open fired. The air filled with hot lead and sparks, as the bullets bounced off the glass. Measuring the glass' integrity with her sensors, GLaDOS calculated that it would shatter well before the turrets ran out of bullets. A lesser AI might have panicked, but GLaDOS had been preparing for something like this for a long time. After all she had been through, she had formulated the means to destroy Chell, Wheatley, Black Mesa and all the turrets in the world, all she had to do was deploy it.
The roof of the chamber began to stir. With the hiss of hydraulics, the ceiling opened up, revealing nothing but blackness. Were they not so deep underground, the turrets might have thought they were seeing the night sky for the first time. Some of the more timid turrets ceased firing and looked up nervously.
With a rustle of feathers and a cry which pierced through the remaining gunfire, a colossal murder of crows descended on the rebellious weapons. The turrets staggered as the birds viciously mauled their wiring, pulling anything they could from the gaps in their weapon compartments, if not simply pecking out their glassen eyes. Some of the turrets fled and others fired desperately into the sky, very few continued their attack on GLaDOS.
The skirmish lasted several minutes, with massive casualties on both sides, until eventually, with a burst of sparks, a beak full of wires was pulled out of the back of the last turret's head. Amidst the smoking wreck of white plates and circuit boards, beneath a heavy layer of black feathers, the remaining birds foraged, pecking at the worm-like wiring.
"Excellent work, my little killing machines." GLaDOS remarked, having regained her composure, although notably not lifting her cracked glass shield.
A small, metallic limb reached from the chassis and picked up Luke's smoking remains from the ground, as GLaDOS scanned him. Most of his circuit boards were fried, but with Rattmann's blueprints, she could easily rebuild him.
"You're not getting off that easy..." GLaDOS whispered menacingly to the unresponsive husk. "I don't imagine your insane creator knew about the room where all the robots scream at you... So you will be in for a surprise..."