Compared to your first battle with Necrozma, the battle against it as Ultra Necrozma is far more difficult. Without a doubt, definitely save before attempting to battle it. With that being said, it would also be a good idea to to have all of your Pokemon at or near Level 70. This is because not only will Ultra Necrozma be fought at Level 60, but it will also have every last one of its stats boosted by one stage as the battle commences.
Since all of its stats will increase, be prepared to be outsped by this thing since the word go. If you have any, it is always recommended to hit Necrozma with Dark Type Pokemon and Dark Type Attacks. Its move, Photon Geyser, cannot cause any damage to Dark Type Pokemon. Plus, Ultra Necrozma is bitterly weak to these types of attacks. Ice Type Moves also help out here, but not coming from Ice Type Pokemon as Ultra Necrozma destroys them with Power Gem.
Most players recommend using Zoroark as a possible lead Pokemon. To start, its ability is Illusion, which has been said to trick the game into having Ultra Necrozma use Photo Geyser over and over. Since Zoroark is a Dark Type Pokemon, this attack will never cause damage to it. However, should it instead unleash one of its other attacks, Zoroark is as good as a goner.
Your best bet when battling Ultra Necrozma is to always come at it with everything you have offensively, having plenty of Recovery Items to heal your fallen Pokemon. That's it. Don't be surprised or saddened if you have to fight this thing multiple times, as most players do since Ultra Necrozma is completely overpowered in this battle. It's okay. As long as you finally get lucky enough to defeat it just one time, that's really all that matters.