Ash has come a long way in his journey to becoming a Pokémon Master. This has been made especially apparent in the latest seasons of the anime. Not only has he been participating in ranked matches with Trainers around the world, but he's been ranked among the top eight Trainers out of thousands. This sounds impressive, but it also sounds too good to be true.
As great a Trainer as Ash has become, it doesn't seem like he should be best-in-the-world great. This rapid success becomes even harder to believe when considering just what he did to rise through the ranks. Between how quickly he climbed and how he did it, it can feel like his success came a little too easily.
Part of what makes Ash's success feel odd is where he started in the rankings. By beating just one relatively low-ranking Trainer, Ash was able to bump up his own rank all the way up to 3,763. In a competition of over 10,000 participants that doesn't account for past achievements, this seems unlikely.
Ash's success can be further attributed to his large winning streaks. When he was just starting out, he was able to rack up six consecutive wins in ranked matches. This caused his rank to go up to 890; by entering the top 1,000, he also advanced to Great Class.
Ash only suffered one brief losing streak during the tournament. After losing to Bea and two more Trainers, Ash's rank dropped back down to 1,031, which placed him back in Normal Class. However, he was able to make up for these losses off-screen and even have a rematch with Bea back in Great Class; since this match ended in a draw, neither side's rank changed. So far, these have been Ash's only known losses in the World Coronation series.
Following Ash's match with Bea, he was able to rack up at least 10 more wins on-screen. This caused his rank to skyrocket from Great Class to Ultra Class (the top 99 Trainers) all the way up to the Master Class (top eight). Despite what should have been a drastically-increasing difficulty, Ash never seemed to suffer a single loss.
On paper, Ash's Master Class position seems well-deserved. To accrue as many wins as he did in ranked matches is no small feat. The fact that he was able to beat so many high-leveled Trainers should be proof of how good he himself is. He almost makes the journey to be the very best look easy.
However, it's precisely because of this apparent lack of difficulty that his ranking is questionable, starting with who he fought. Despite the World Coronation Series apparently being a competition to prove who's the best of the best, Ash only fought a handful of noteworthy opponents. Most of the big-name characters of the Pokémon franchise didn't start appearing until Ash was in the Ultra Class. Until then, he was mostly battling characters of the day in easily-won battles.
Speaking of battles, that's another thing that might have been too easy. Most of the time, Ash only had to fight battles with one to three Pokémon. Even in the Ultra Class, he was able to advance after a one-on-one match with Marnie. These kinds of battles are standard for the anime, but if the goal is to see who's the best Trainer, they probably should have fought with more Pokémon.
Ash's selection of Pokémon is perhaps the hardest thing to accept about his win streak. With the obvious exception of Pikachu, he's relied solely on his five most recently-caught Pokémon to get him through the entire World Coronation Series. If he wanted to use them in the lower-ranked battles, that would be one thing, but if he means to be the best in the world, he could have at least considered some of his older, stronger Pokémon for use. Unless his current team is somehow the strongest one he's ever assembled, it shouldn't be this easy for him, to advance.
With all of that said, perhaps there are ways to justify Ash's success in the World Coronation Series. Even if he's still 10, he's already had a lifetime's worth of adventures and battles with Pokémon. his experience should explain how well he's doing if nothing else. Looking at it that way, his ability to challenge and beat Elite Four members and even Champions seems like a natural progression in his story.
His ability to handle himself in ranked battles is also commendable. Doing this requires not just power and skill, but also the mental fortitude to maintain one's composure in a match with palpable stakes; if he'd lost even one match, it could have set him back pretty far. He could have easily lost his confidence and, with it, his ability to fight, as was illustrated by his losing streak. Without confidence in his abilities and the resolve to power through the tough times, he wouldn't have gotten far at all.
Furthermore, Ash's team, despite being made up almost entirely of new Pokémon, is a powerful team, nonetheless. Obviously, his Pikachu has been through about as much as he has. His other Pokémon like Dragonite, Gengar, and Dracovish are particularly heavy hitters, even by the games' standards. Additionally, his Lucario is a contender to be one of his strongest Pokémon ever; it might even be up there with the likes of Charizard and Greninja. As powerful as his older Pokémon may be, his new Pokémon shouldn't be underestimated.
Even the lack of Full battles is justifiable. In truth, Full battles are rare on all fronts when it comes to Pokémon, be it the games, the anime, or the manga. Shorter battles with three or fewer Pokémon are commonplace because they streamline the process and allow for both more battles as well as more stories. Even in battle facilities, the hardest challenges of any mainline Pokémon game, the only way to use more than three Pokémon is through Double, Triple, and Rotation Battles. The use of three or fewer Pokémon in the World Coronation Series thus far is simply a reflection of how most battles in the anime are conducted.
Furthermore, the real World Coronation Series could be said to have started in the Ultra Class. This is where Ash started regularly battling important Trainers like Marnie and Volkner. By the time Ash had reached this Class, all of his Pokémon were reasonably well-trained and it made sense that they could hold their own in these genuinely hard battles.
With all of that said anything could happen in the Masters tournament. Ash could still use more of his older Pokémon and the format could still be changed to six-on-six by the final round. This doesn't necessarily make up for the rest of Ash's relatively smooth sailing, but it would at least make the difficulty of this ultimate tournament more believable.
Whether or not Ash's advancement through the World Coronation Series was a contrivance, the fact remains that he's a top-tier Trainer. For all the time he's spent journeying through the Pokémon world, he should have the experience necessary to contend with even its greatest Champions. As for whether he should be able to do it with the small selection of Pokémon he's been using, that's still up for debate, as is whether he can win it all.