How to catch the legendary Zacian and Zamazenta
After winning the Champions Cup against Leon, you will be given the credits table, which makes the game feel like the game is over. However, Pokemon Sword and Shield is far from over at this point. You start at home and get a visit from Professor Magnolia, who offers you a Master Ball.
You will find yourself on a quest to resolve the Dynamax issue started by two new antagonists that will be introduced. The game lets you know exactly where you need to go through this process on your map. Through this process, you will eventually find yourself at the Hammerlocke Stadium and the Powerhouse where you have already caught Ethernatos. Talk to Sonia and enter the elevator and follow the sequence of events.
Eventually, you'll end up meeting the legendary opposite for your version, Zamazenta in Sword and Zacian in Shield, who you must fight. This fight shouldn't be too hard as long as you keep up with your type matches. For this match, Zacian will be Combat type and Zamazenta will be Fairy type, as this is their non-weapon version. However, this Pokémon will be level 70, so don't bring a bunch of grossly underqualified Pokémon. Once you have defeated the legendary opposite, your legendary version of Legendary Pokemon will appear and ask you to fight it. This allows you to heal and save before the fight, so it's a good idea to do both before interacting with the Pokemon.
Before all of this, it's a good idea to prepare with any supplies you might need for it. Resuscitation and Full Restores or Max Motions can still help, but the most important here will be what type of Poke Ball you are going to use. You can just go ahead and use your Master Ball which has a 100% catch rate, or you can be like me and prefer the challenge of catching it with another type of Poke Ball for fun. If you're looking to go, your best bets are the Ultra Ball and the Timer Ball. Timer balls will be your absolute best bet if this fight is going on for a long time, so grab a few.
If you're going to use a Master Ball, throw it at the first corner and you'll catch Zacian or Zamazenta. For those looking to work hard to catch it and save the Master Ball, be sure to bring someone who can inflict Paralyze or Sleep on the Pokémon. While Sleep adds a 2x multiplier to the catch rate and Paralyze only adds a 1,5x multiplier, I prefer to use Paralyze because it actually remains the rest of the battle after using it just once. This not only slows him down to where you can attack first each turn, but it increases the capture rate. Sleep only lasts a few turns and may be more difficult to get out of. For Paralyze, Nuzzle comes highly recommended as a very weak electric type move that paralyzes and deals damage, so you can also help lower the health bar later on.
Compared to the previous showdown, this version of Zacian or Zamazenta you'll face is the powered-up weapon version. This means that Zacian will not be a Fairy / Steel type and Zamazenta will be a Combat / Steel type. Using fire or ground attacks will be super effective against both, so that should be your goal, although you should also make sure you don't knock them out with either of them.
Once you have Zacian or Zamazenta down in health and have a Paralyzed or Sleep status effect, start throwing Ultra Balls at him if it's at the start of the battle. If it's taken a while, start throwing timer balls. If you're about to lose and haven't caught it yet, you can still go out there with your Master Ball and guarantee the catch. Whichever method you use, your exclusive Legendary version is now yours. To get the other exclusive Legendary version, you will need to trade with a friend who has the other game.