Except this isn't a double or triple battle. These are all 1 v 1 battles. This depends entirely on the skill of the trainer. All of Ash's regional teams have type and form (aerial and ground-based) diversity, and unlike Paul, Ash relies on counterstrategy/improvisation rather than setting up weather, entry hazards, or other effects for rest of his team to exploit.
It's hard to justify "ultimate [regional] team" when Pikachu and (Sato)Smogonfrog dominate the show. It's sort of how people claim DP had the best regional team, even though by the end it was mainly Infernape doing most of the work and a bunch of others being steady, but not great.
It has nothing to do with a double or triple battle. Look at the match with Sawyer. Hawlucha was defeated by Slaking and Talonflame came into the battle to pick up where he left off. While someone like Hawlucha uses his opponent's moves to his advantage to gain momentum and power, Talonflame uses its speed alone. In doing so, it overpowered Slaking and brought home the victory. Same case with Talonflame vs Clawitzer. The bird was defeated only for Pikachu to come in. In all of these matches no one has gone down easily for Ash's team as they've all taken hits and lasted. Of course, people will argue against it no matter what.
Now, how are Pikachu and Greninja stealing the show? Prior to evolving Greninja held next to no focus and it was mainly given to the birds and Goodra at the time with a little Pikachu sprinkled in after his staring in the first three gyms. But, from the fifth badge up until the ninja village episodes, the main focus remained on the three fliers and Serena's team. Now, in the League Pikachu has 2 wins and will be battling at least four of Alain's team. But, look at how he isn't getting the wins and Ash can say "Hey, Pikachu isn't working. Noivern will be better for Tyranitar." or "Hawlucha's speed is great for Weavile". Everyone is getting a shot to perform as a
Team unlike past seasons where there was the Charizards or the Sceptiles. Greninja meanwhile has had only two battles in the entire League and Goodra has had more screen time since XYZ 30 than Greninja has in that same span.
So, a little anology for all you baseball fans on here:
Ash's team is a great pitching staff, without one piece they'll fail. If the starter doesn't pitch well then the rest of the staff is struggling to keep up.
Pikachu = starting pitcher. He's the most reliable member of the team. Someone you can call upon in any situation to be a stopper. He stared in the first 3 gyms before turning to a supporting role up until the League where he took out two of Sawyer's team, changing the tide of battle.
Hawlucha = Set-up man. Time and time again he's come through in tight situations to get the victory for Ash. Heliosk, Laverre City, Astrid, and now hopefully against Weavile.
Talonflame = Middle Reliver. The first pitcher you turn to when your starter is tiring. releiable and dependable where he can get leftys and rightys out at the same time. In the sense that Ash uses him in almost every gym match and is constantly using him in important situations in battle. Has shown to be able to beat Pokemon in both the sky and on the ground through it's speed and physical attacks.
Goodra = Long Reliever. Someone who can come in and give you three innings on a bad day. this
is mainly based around Goodra's bulkiness and how he can tank hits. Without this stopper than who does Ash use to stop powerful attacks?
Noivern = LOOGY (Lefty One-Out GuY). A situational pitcher who you bring in to get one certain batter out in a tight spot. Not the best, but he's able to get the job done when called upon. Case in point: battles with Metang, Doublade and Salamence where it lost one, won one and tied.
Greninja = Closer. the one the manager turns to in the toughest situation of any game, the last inning. With your team tied or holding a slim lead, using arguably your best pitcher to seal the win is a no-brainer. Same case with Ash using Greninja at the end against the strongest members of his opponents team.