After defeating Ultra Necrozma, you will be permitted by the Ultra Recon Squad to continuing using there Legendary Pokemon to travel through Ultra Space. As you do this very thing, you will come across a number of Legendary Pokemon from previous generations as you travel further and further in the Ultra Wormholes. Like all Legendary Pokemon Encounters, these Pokemon are among the toughest to capture.
Each and every time you find and battle a Wormhole Legend, it will be at Level 60. Outside of the Legendary Pokemon Landorus, all of these Legendary Pokemon can be paralyzed with the the move Thunder Wave, thus making each of them easier to catch. Since Landorus is a Ground Type Pokemon and thus cannot be paralyzed, unfortunately, you will have to go without status conditioning it during battle.
After paralyzing each of these Legends, try to lower their HP as low as you possibly can. The move False Swipe works great when a Pokemon doesn't have much HP left, as it will always leave the Pokemon with at least one HP. The lower a Pokemon's HP, the better chance you have at catching it. If you have the Rotom Power that makes it easier to catch a Wild Pokemon, then you should definitely consider using it for these given Legendary Pokemon.
If you still have your one Master Ball, you can use it on any of these Legendary Pokemon; however, Ultra Balls are usually the best type of Poke Ball you can use on all of them. When you enter a Wormhole that holds a Legendary Pokemon, you will always be notified of its presence. After receiving such a notification, ALWAYS save your game. You only have one shot at catching each of these Legends, and this safety precaution has been working ever since the first generation of Pokemon.