The “final” legendary pokemon is Necrozma in Pokemon Sun and Moon, who requires quite a bit to capture. Even though the requirements are high and he isn’t that good, this guide will help make it a little easier.
To unlock the ability to find Necrozma, you first need to complete the story, which also includes defeating the Elite 4 and then finish the ultra beast side story. After all of this, Looker will tell you of a mysterious sighting over Melemele Island and this unlocks the ability to find Necrozma.
Pokemon Sun and Moon Guide
To find Necrozma, you first need to head to either Route 1 or your house. From there, head to the sea and take a left at the beach. There you should find a cave that requires Tauros to enter. Run through the cave until you find an opening at the other end. Please note, I strongly suggest heading down the Machamp path to get a new Z-Power.
Once outside, you’ll find Necrozma randomly in the wild. To speed up the process, use a repel. This will make it so only Necrozma is the only possible encounter.
While Necrozma isn’t powerful, he is extremely difficult to capture. However, there are two things you want to keep in mind. The first is that defeating him will simply cause him to run, meaning you can fight him multiple times without restarting if you want and he is not considered an ultra beast, meaning beast balls shouldn’t help you.
Speaking from my experience, I found Necrozma was easier to catch with worse balls. This could be dumb luck, but a great ball had better results than an ultra ball and I actually ended up catching mine with a beast ball, something people claim is difficult to do. This isn’t bragging as much as suggesting you might want to try other things before resorting to a master ball.