How to Unlock Dialga and Palkia
Near the end, you’ll be kicked out of Jubilife Village and must go and collect the pieces of the Red Chain. You’ll be asked who you want to accompany you on your journey: Adaman of the Diamond clan, or Irida of the Pearl clan?
At the end of the mission Atop Mount Coronet, you’ll come across the first one and fight it. After the fight, it will become clear you got the wrong one and you’ll have to go and get the other after crafting a special Ball to catch it: the Origin Ball.
How to Beat Dialga and Palkia
For the first Pokémon you fight, you’ll have to attack it just like you would a Frenzied Pokémon and then battle it when its health is whittled down. For the second, you’ll only have to do the Frenzied portion and won’t have to actually engage in a Pokémon battle — you’ll automatically throw the Origin Ball at the end — so, really, you only have to fight one of them and it’ll be the one you happen upon first.
The Dialga FIght
The goal is to get down the health enough to start throwing Ultra Balls and catch it (as Great Balls are unlikely to do the job). Using a status move can really help, though it’s always important to beware of conditions like poisoning and burning which could make the Pokémon faint.
The Palkia Fight
If you’re up against first, it will be Level 65 too. Throw the balms and dodge attacks in the same way you did Frenzied Pokémon to whittle down the top bar to the bottom and then it’s time for a fight.
Much like Dalgia, you’ll want to bring down the health to the red and throw Ultra Balls while being careful not to make the Pokémon faint. A move like Thunder Wave or Hypnosis is also very handy for making the Pokémon easier to catch.
Make sure you’re loaded up on plenty of Balls for either one!