Hoenn.
This wasn't the first time they had entered the Battle Pyramid. As they stepped through the doors, Brock's mind replayed echoes of the previous times they had come here. Neither Sceptile's strong stoic stabs nor the warping heat of Torkoal's flames had been enough to overcome Brandon. The man had three legendaries, for Arceus' sake.
But Brock knew, better than most, that Ash wasn't the sort to succeed on the first try. It was a good thing he wasn't the sort to give up, either.
If Ash had less perseverance, he never would have even made it past the gym battle against Brock himself. He recalled Ash's unusual tactics, using an electric-type Pokémon to win, despite the disadvantage. That had been Brock's first clue that the boy in front of him (preparing for battle again) was no ordinary trainer. True, Brock had only lost the gym battle because of the errant sprinkler system, but Ash no longer needed artful (almost unfair) tricks to win. He had increasingly been earning every Gym Badge and Frontier Symbol with his power and sharp tactics.
Brock had his thoughts interrupted by a belch of fire into the sky from Charizard (this time, on Ash's command). The referee commanded the battle to start.
Brock had never been interested in the nuts and bolts of battling, preferring instead to focus on strategies. His mind began to wander as the Pokémon traded blows. It seemed that Charizard had significantly increased its power since Ash left it in Johto. It had a few new moves (which offered more flexibility, in his opinion), presumably learned in the Charicific Valley. How was Liza doing, he wondered…
His mind was jerked from a daydream when Charizard collapsed to the ground, adding itself to Brandon's list of victories. It had simply been outmatched (legitimately, unlike its loss in the Indigo League). This put Ash in a tough position. One of the most powerful weapons he had was gone, but Brock had learned to never count Ash out.
Indeed, Ash recovered under pressure, using Bulbasaur and Squirtle just like in the Kanto days. His strategy was solid, his attacks hit hard, and he slowly regained the advantage.
Brock scrutinized Ash, chest blooming as if he was looking at one of his many younger brothers. He was proud he'd gotten to see a capable battler emerge from the chrysalis of a lucky rookie, over the years. It had a been a long journey worth every forest-step (even when they got lost). Moreover, who better to actualize Ash's metamorphosis than his original gang of four starters?
"Use Solar Beam."
"Bulbasaur, use Solar Beam!"
Brandon and Ash spoke in harmony; two beams of light clashed in the center of the arena. Shadows were flung around like ragdolls by the sheer force of the attacks. Where the beams met, a phosphorescent ball of light grew steadily. Brock narrowed his eyes.
When the dust settled, neither Pokémon was standing.
Only Pikachu and Regice remained; Brock had anticipated that it would come to this. Ash's oldest partner against a legendary Pokémon. On paper, Brandon was dominating and would surely win the battle (but this wasn't on paper). It was far from over.
Brock knew the Pyramid King wouldn't underestimate any challenger. As if on cue, Brandon began to methodically trap his opponent. Brock shivered in the blizzard as the arena began to turn hostile, the air filling with hail and the floor freezing into ice-slips. Pikachu got a few hits in-between moves, but it was clear he was losing his grip on the arena (and the battle). Pillars rose to cage Pikachu; Brock had to admire Brandon's strategic play.
"Regice, use Rest." boomed Brandon's gruff voice.
At first, it seemed like nothing was happening. Brock waited. A moment later, he watched, mesmerized, as the scars on Regice's body meshed themselves back together (mesmerizingly), as if it were his own hands using a Potion on them. Regice had healed completely, leaving Ash far behind.
It was undeniably a bad situation. Nevertheless, it still hadn't seemed hopeless. That is, until Pikachu was hit squarely by an Ice Beam. Frozen solid, unable to attack, even Ash's greatest partner was helpless to continue the fight. Ash called for another bolt of inspiration, but he was left unanswered. Brock wanted to walk down, to advise Ash to forfeit, but the look on his face rebuffed any such thoughts.
Brock couldn't see a path through the icy defenses. He turned to see Ash: his jaw was set harder than the ice of Regice's body. He was going to persevere (of course). There was a moment of silence.
The referee lifted a flag to declare the battle over.
Then – incredibly – the ice began to glow.
A yellow glow surrounded Pikachu, emanating from the chunk of ice he was trapped in as if it was glittering gemstone. One crack. Then two. Gradually, then suddenly, all at once, Pikachu burst out of the ice with electric speed. The towering Regice lifted a monstrous icicle-arm to blast Ash's small mouse (dodged); a trail of sparks crackled in the air as Pikachu raced across the arena, seemingly defying the icy floor, defying the referee, defying all of Brock's logic and strategy (not for the first time). Pikachu hit Regice head-on, catapulting it backwards into the stands like a thunderclap. There was a sudden silence, broken only by the soft buzzing of electric bolts.
Pikachu landed and shook the stray ice off its cheeks.
"Regice is unable to battle!"
The referee's voice echoed around the empty arena.
Brock needed a moment to take it in. Ash had defeated a legendary Pokémon. He had won the Battle Frontier.
Brock peeled his eyes away to see Ash lift the Brave Symbol, a smile of genuine victory across his face. They left the pyramid, as winners, at last.