This is a romanticised version of how I beat Dialga in Pokemon Mystery Dungeon: Explorers of Sky. Kudos if you spot the reference...

Focus Punch

"Art...we're finally here."

Art the Riolu, leader of Team Soul, looked at his friend and partner, the Eevee known as Sam; she looked tired after climbing the so many hellish floors of Temporal Tower. Her Red Band, tied around her neck, had a tear in it. However, in her eyes shone the bright light of determination, and the Riolu nodded at her shortly, the same implacability in his own eyes. Together, the two young founders of Team Soul stepped forth from the stairs and forward along the roof of the Tower.

"Wow...look at the sky, Art!" Sam pointed out in wonderment; "Why..why's it all red like that? And all that lightning! I..I think we need to hurry!" Her last statement was accompanied by a slight tremor, and Art picked up the pace; he couldn't provide her the answer as to why the sky was the way it was, but he more than agreed with the need to make haste. He jogged forward, Sam following, and ahead of them a strange sort of altar-like structure loomed; it was blue, intricate and pretty, but had five unsightly hollows in it...

Wait. Five...hollows...?

Sam voiced his theory when she murmured, "Art...look at that...I wonder what it's for..?" She gasped. "Wait! It's there, the Time Gears fit in there! Come on, Art, let's do this quick!" Again, the quiet Riolu nodded and both friends started to scurry closer to the altar, when-

"GROO-OOOOHH!"

"Aaah!" Sam yelled out as the both of them were pushed back a few paces by an invisible force and the area around them darkened; Art couldn't see a thing, and squinted through the shadows as they both got up. That roar...it sounded strangely familiar. But from where..?

Then he remembered, and dread filled his heart. The last time he had heard a roar like that was in the dark future. It was the roar of...of...

"What's happening?" Sam exclaimed fearfully, and a deafening voice roared in response, "YOU! ARE YOU THE ONES DESTROYING THE TOWER?!"

The light returned to the area, and before them in all his terrible fury stood Primal Dialga. He was breathing lowly, heavily, glaring at them with frenzied rage in his eyes. Art didn't like those eyes; they were far too ferocious, too feral in nature. It was unsettling. "No!" Sam protested; "We're here for-"

"SILENCE!" Dialga roared. "I WILL NOT TOLERATE INTRUDERS OR VANDALS TO THE TOWER! I WILL PROTECT THIS PLACE, NOW BEGONE!"

"No! We're here to restore time, honest!" Sam pleaded, but Art saw the Legendary's eyes cloud over; it was hopeless, he realised. The beast had fallen too far into the darkness. "TIME...?" Dialga questioned, his voice taking on a lost quality, like the very thing he represented was something alien to him; "TIME...I MUST PROTECT...TIME! GROOO-OOOOH!"

"It's no use, Art.." Sam lamented; "He's fallen too far, he won't listen to reason!"

Art thought deeply about the situation at hand. Yes, Dialga seemed beyond help...but he also was nowhere near as monstrous as the Dialga in the future. This Dialga could still form coherent words. Perhaps...if time were to be restored quickly enough, he would regain his reason. But so long as he stood in-between them and the altar, that wasn't happening anytime soon. And neither he nor Sam had the capabilities to take on Primal Dialga; they weren't even at half strength, they were tired and they frankly didn't stand a chance.

But Art had one move which, if he put all his power into, might work...naturally, it might also be the end of him. But a voice in his head reminded him that sooner or later her was going to fade anyway; at least this way, he would disappear on his own terms.

As he grit his teeth and steeled his nerves for the end, Sam stammered, "H-here he comes, Art!"

He looked at her; a real, last good look at her. After this, he would never see the Eevee again. She was, to him, his first, best and maybe even his only true greatest friend. She meant more to him than anyone else back at the guild, more than the other members of Team Soul...more than anything in the world. Sam was his world.

As the raging Legendary prepared to charge at them, Art closed his eyes against a sudden rush of tears, clenching his fist. He didn't want to leave her alone, but he knew it was inevitable after what Dusknoir had told him and Grovyle before both Pokemon from the Future left. Sam had to be strong.

For the world. For...you, Sam.

A light blue aura generated around him, flowing upwards like a great flame, and Sam let out an exclamation at this new development; "MY TOWER!" Dialga howled as he started to thunder forwards, but he seemed slow, as if time had turned to mush around them. Every detail became easily viewable to Art as he narrowed his eyes at Dialga; "Dialga!" the Riolu shouted, "the way this is going to end seems becoming of you and me...I'm going to give back to you what you lost, using my life!"

Sam's eyes widened; "Art?!" she yelled hysterically, not fully understanding what he was saying as she stepped back a little from his steadly growing aura. Art merely sighed and smiled sadly sideways at Sam; "Sam, I'm sorry, but...this is goodbye! Finish it, for us! For Grovyle! And because only one who can surpass me can lead Team Soul...you are the new Team Leader, Sam!"

The Eevee looked stricken, starting to tear up as she realised what this meant. Art nodded at her a last time, then turned his full attention on the charging Dragon/Steel type; he then ran at Dialga, planning to meet the Time God at a running speed. "I CANNNOT DIE! I WILL NOT DIE!" Dialga roared, his sense of self-preservation already becoming prevalent, but Art knew what he had to do. He propelled himself towards Dialga off his feet into the air, pulling back his right fist as the blue aura around him reduced and focused around the same fist.

Sam watched with tears in her eyes as Art yelled, "Focus...PUNCH!" and struck Dialga in the face with his aura-emblazoned fist. Such was the force of the attack, Dialga's charge was fully interrupted and a white light enveloped the area; "ART!" Sam screamed in grief as the light exploded outwards, and the Riolu himself watched with fading strength as he hung in the air a bit, staring bravely into the white explosion he created moments before it engulfed him and Dialga exclaimed in disbelief and surprise.

From the top of Temporal Tower, and viewed for miles around by countless Pokemon, a bright white beam lanced into the sky and continued to shoot for what seemed an eternity before it faded away. As Sam opened her eyes and got off the floor, she blinked at the sight of Dialga unconscious, and the red clouds in the sky momentarily parted. Art was nowhere to be found.

The Eevee sniffled pitifully upon realising that her best friend was no more, but managed to get her act together enough to stumble over to the altar, ignoring Dialga and the slight tremors, and place the Time Gears in their proper places. As yet another bright white light enveloped the area and the Time Gears did their work, Sam collapsed into a forlorn heap and wept for her lost noble, heroic friend.

Oh, Art...