How to Unlock Giratina
As with most of the Legendary Pokémon, you’ll have to beat the entire game before you can unlock Giratina. However, Giratina is actually one of the last Pokémon you can encounter and usually comes right before finding and catching Arceus.
You’ll have to complete all 240 Pokédex entries before you can access this Pokémon.
You’ll also have to gather all of the Hidden Plates and face Giratina in battle (without the ability to capture it quite yet).
Giratina is a type so it can be quite tough to take in battle. Although you’ve defeated the game and should have some pretty high-level Pokémon by the time you take it on for the first time, you’ll still have to take some care.
Giratina is weak against Dragon, Ice, Ghost, Dark and Fairy-type moves. Gengar, for example, is a good counter to take it on, but you should make sure you have a couple lined up in your party just in case. It’s always a good idea to manually save before you enter a battle like this, just to make sure you can have another shot right from the starting point if you need to.
Once you’ve completed the Hidden Plates, including beating Giratina, talk to Professor Laventon by the gate in Jublife Village. He’ll tell you he’s seen a huge shadow you can go and check out, unlocking Request 91: On the Trail of Giratina. You can guess what this huge shadow is.
Find Turnback Cave by heading to the Cobalt Coastlands and going to the northwest part of the map. Inside of it, you will find Giratina.
How to Catch Giratina
Since you’ve already fought Giratina, you’ll be familiar with the moves it’s weak to and should use the same counters again. However, this time, you have to be careful not to make the Pokémon faint, so your goal is a little different. Adjust strategy accordingly!
Manually saving before entering the battle is, again, key. That way if you do go too far, you can restart.
Whittle down the Pokémon’s health to the yellow bar — preferably the red, though it’s riskier in terms of fainting — and start throwing those Ultra Balls. Make sure you have plenty though in a pinch you might be able to catch it inside of a Great Ball too.
Using status moves is another great idea, though poisoning and burning are likely to keep depleting its health until it faints. Instead, try for sleep and paralysis, as this is more effective in making the Pokémon catchable.
Once you’ve done that, you can move onto catching Arceus, the last main thing to do in the game.