In a plot twist completely not ripped off from the main series Pokemon games, your rival has already beaten the Grand Masters.
This time however, he has already claimed the Legendary Pokemon Cards for himself.
Articuno, Zapdos, Moltres, Dragonite. One deck.
And you have to beat him to claim the cards for yourself.
Not only does he have the four legendary cards, he also utilises the three Eevee evolutions too.
The odd thing here is that he only uses Fire Energy.
His Articuno and Zapdos are played purely for their Powers, and can't attack.
His Eevee evolutions all use Colourless energy however.
His diverse typing means he doesn't have a particular weakness throughout his deck, but there's a few types you can use to not have ANY weaknesses.
Water types not weak to Lightning, Grass types not weak to Fire, Gastly line, Colourless, and Lightning types can all be used.
Lightning types and bulky Colourless are probably the way to go.
Snorlax can resist status, hit back, and let you stall to evolve Pikachu.
Electabuzz will be hitting off the bat too with Thundershock, and then Thunderpunch.
Poliwrath alongside Articuno/Gyarados would work well too.
Poliwrath isn't weak to anything, and has the ability to remove energy whilst doing solid damage.
Articuno can hit the bench whilst dealing solid damage, and Gyarados can deal solid damage, provided you can evolve it.
Focus on the Dratini/Dragonair if they pop up like with Rod, and the rest should take care of itself. Lure in any Articuno/Zapdos, and Ronald will be forced to take a hit, or, waste energy/switch cards to get them out of battle.
After beating Ronald, he loses the four Legendary Cards, and you can go into the back room to claim them for yourself. They speak to you which is kind of odd.
You also can learn the configuartion for the Grand Masters decks, Ronald's, and one other.
Time to watch the credits I guess.
There's two things left in the game.
Collecting all cards, which in turn requires you to replay the Challenge Cup, do some trades, and there's also the Challenge Machine which puts you up against 5 random enemies in a row with the same deck.
I won't be covering any of that, because it's kind of pointless rebattling opponents for no story element involved, and no increase in difficulty.