How to Unlock Shaymin
First of all, to unlock Shaymin, you’ll need a save file from Pokémon Sword or Shield on the same console as the one you’re playing PLA on. This is a vital step.
Like most of the other Legendaries, Shaymin can be found in the post-game (as long as you have this save file). Once you’ve finished all of the main missions, you can go to the blackboard in the Galaxy Headquarters in Jubilife Village and access Request 92: A Token of Gratitude.
Speak to Medi as the quest dictates and she will tell you about a mysterious Pokémon and give you a Gracidea Flower. You’ll then have to head to the Obsidian Fieldlands and go to the Floaro Gardens west of the Fieldlands Camp. Find the patch of flowers that matches the one Medi just gave you, which should be pretty easy to spot due to the quest markers. You can use Braviary to get there quicker.
Once there, Shaymin will appear just like any other Pokémon would. It’s rather small compared to most of the other Legendaries you’ll come up against in the game!
How to Catch Shaymin
You have two options for catching Shaymin.
Unlike many of the other Legendaries, you can throw some Ultra Balls and see if they stick. Great Balls and Poké Balls aren’t very likely to do the job since Shaymin is Level 70, so make sure you’re stocked up before you head there.
You can also choose to throw a Pokémon at Shaymin and battle.
Shaymin is a Pokémon. That means you can use Fire and Flying-type moves to whittle down its health quickly, though as with any Legendary, be careful you don’t go too far. If you make Shaymin faint, you won’t be able to throw a ball and catch it, so you need to pick your attacks carefully.
As Shaymin is Level 70, you shouldn’t have to worry too much about a one-hit KO unless you have a very high level Pokémon, but it’s still important to be cautious.
Using status moves can also make this Pokémon easier to catch. Moves like Thunder Wave and Hypnosis will render Shaymin paralyzed or drowsy, which makes it harder for it to break out of the Balls you’re throwing.
It might be a bad idea to poison or burn it though. Doing this means that its health will slowly deplete every time it breaks out of a Ball so if it manages to break out a few times, it could accidentally faint. This is the last thing you want to happen!