I beat it in one try but it was clutch. I let it knock out all my Pokémon with Photon Geyser, then restored two of them as I let my Mudsdale stall for a bit. I have to thank my Primarina's Moonblast for this victory.

Never spent so much time trying to figure out how to win a battle before this xD
At the first tries, my team got wiped out entirely, I kept grinding up til all my mons were at least lv. 60, then added Zoroark to my team
That helped a lot

Step 1: Have Mimikyu in front.

Step 2: Ultra Necrozma used Smart strike, which was of course protected.

Step 3: Mimikyu used Toxic.

Step 4: After Mimikyu fainted I summoned my Ferrothorn which used Leech seed.

Step 5: Stall for few turns.

Step 6: Dead Ultra Necrozma.

There all done. Though I’m pretty sure that won’t fly in a nuzrun tho, so I might have to think of somthing different next time.

There are only two consistent ways of beating Ultra Necrozma, from what I gather.

Bring in a Steel type with good Special bulk that resists its whole movepool. I used a Bronzong for this, but that requires having either a Gen V game or OR/AS and trading it up from there. Going by base stats, alternatives for this strat include Metagross and Magnezone; who can shrug off nearly any hit bar crits off Photon Geyser (in Magnezone's case) after having X-Sp.Def used on it. Bring the Steel type into the fight and boost it up with X Sp.Def so it can stand up to Necrozma's attacks. And then continue the fight like normal; attacking and healing when your Pokemon's HP gets low.

The other consistent strategy (and the easier one of the two, if you didn't train up a good Steel type) to do, and is probably something Gamefreak added at the last minute after their testers ran into issues with the fight, is to catch a Zorua at around the Trainer School in Route 1 and level it up until it learns Foul Play (Level 29). Then take it into battle with Ultra Necrozma and put it out in front, with a pokemon weak to only Photon Geyser in the last slot of your party, and spam Foul Play until Necrozma falls.
The result is that Zorua/Zoroark transforms into an illusion of said pokemon weak to Photon Geyser and Necrozma spams the move on Zorua/Zoroark, doing no damage and not getting the message that it should use other moves, while you knock it out with Foul Play. Surely Necrozma's fatal vulnerability to this strategy was a last-minute nerf to the pokemon during testing.

The reason I am not listing any strategy involving Focus Sash is that it is consumed upon usage. If you by chance used it in an earlier encounter or you want to save it so you don't have to drop 48 BP on another one, you are not always going to have one on hand to use in the fight.

I appreciate making the game harder, but the Ultra Necrozma battle is just poorly designed because beating it demands you to bring in one of a few select Pokemon that happen to be equipped to handle it, have had EXP Share on for the whole game (and trivialize the rest of the game as a result) or use luck-based strategies and hope they work.
Ultra Necrozma could have been just Level 50 (or even Level 55) and the fight would have been fair.

...I'm not sure how to feel about my experience with Ultra Necrozma.
See, my plan was to put the Poni Wilds Focus Sash on my Toxapex, use Toxic and stall it out with a bunch of Revives (44, to be exact). A little cheap, but I could live with that. (:P) It went off as it was supposed to; after Tox fainted, I randomly decided to send out Mimikyu next. I selected "Let's Snuggle Forever" just for giggles, since I knew Photon Geyser negates Disguise...But then Necrozma used Smart Strike instead?!
I got to take off most of its HP with the Z-Move, and then poison damage knocked it out next turn...So basically, I was planning to win by employing a cheap strategy, but instead I won because of the AI making a really stupid move. ~_~

Mine was at Lv. 65 at the time. Mainly because of that Lucky Egg Kukui gives you. :3

I think Ultra Necrozma is either hard af, or easy af depending on what strat you use. For me, he was hard af since I chose not to read up on it beforehand, but I ended up getting lucky and beat it in one try.

I remember leading off with my Incineroar since I figured he was still Psychic type, but he one-shotted it with Power Gem (bruh). So I sent out my Golisopod, hoping to get a nice First Impression. Barely did around 30-40% (bruuh), and he got one-shotted with another Power Gem. ;-; So I went out to Kommo-o for more damage, but then he outsped and it got one-shotted with Dragon Pulse, revealing to me that he was also part Dragon-type (bruuuuuhhh). I then went into Tsareena, and managed to get a Teeter Dance off when it missed a Photon Geyser (thanks Pokemon Refresh). I revived Kommo-o and Incineroar before Tsareena got one-shotted. I sent out Incineroar, then swapped to Kommo-o to bait out the Power Gem. Next turn, it hit itself in confusion, and I managed to take it out with Devastating Drake.

My dudes were like level 54-55. I did not expect a Lv60 monster beast capable of vaporizing teams without even getting out of bed. Then again, I'm glad Game Freak actually tried to provide a really good challenge in a Pokemon game since Colosseum and XD, and I hope this continues, but on less of an Exodia-like scale. I've never thought so much about what I had to do, and it most likely would've downed my entire team if not for that Photon Geyser miss.

OMG guys, I literally had to use my LV. 100 Milotic shiny JUST so I could beat the d*** thing! That sucker literally wiped my team three times, which made it more infuriating.

So I went to the PokeCenter and said, "F it. I'm pulling in the big guns." After needless tries of using Milotic to listen to ME, I finally defeated Ultra Necrozma! XD

I was just sooooo upset, and that is how I beat him.

In my Water Monotype, I beat it today thanks to my Lapras (with a -Sp Def nature no less, but quite a few EVs) surviving its Photon Geyser with 7 HP and using Perish Song. Protect helped on that note, and it did a little over half to my Primarina with Power Gem (who I had a Charti Berry ready on Lapras for) and then OHKO'd my Swampert with Photon Geyser before it succumbed.

Did you find the battle to be challenging? Did you find it to be be easy? Did you use a strategy to beat it?
What do you guys think of this battle?

It was very difficult. I had no idea this would happen, and got destroyed as a result. I loved making up a strategy for this battle, which involved using a focus sash and the move toxic.

hahahaha its difficult but before i battled him i saved the game first but i lost against him 2 times after every lost i rest the game and finaly in the 3th time i beated him by a perfect strategy specials thanks to magnezone he was verry usefull the only advise i can told everyone here is to take a steel pokemon with you coz of his defensif state

https://www.pokecommunity.com/showthread.php?t=403909

https://www.pokecommunity.com/showthread.php?t=404921

https://www.pokecommunity.com/showthread.php?t=404617

Honestly...it was badly-done.

You've just fought a level 50 fused Necrozma, and your team at this point is around level 50-55. As soon as you chase Necrozma into Ultra Megalopolis and the top of Megalo Tower, you'll soon realise how much he outmatches you - he's level 60, has +1 in all stats (which are really high), amazing coverage and a built-in Expert Belt. The game gives you almost no warning about it until right before you go in, leaving little room to grind and prepare for what is probably the most difficult Pokemon battle in the series so far. It's no surprise that a lot of people have died on this battle alone.

It seems that a lot of people, me included, have had to resort to cheap tactics in order to beat him despite it not being what the game intends you to do. A common strategy was to lead with a Zoroark disguised as a Pokemon weak to Psychic, and spam a Dark attack such as Foul Play or Dark Pulse without fear of Necrozma hitting you. Another similar strategy is to use Malamar and either invert his boosts or Foul Play it. But to be honest...the fact you're forced to use strategies like this puts a strain on teambuilding, taking the fun out of a game where you can get through using whatever mons you want.

I feel relieved that I'm far from the only one that thought Ultra Necrozma was kind of a pain in the ass.

... and honestly? I also thought it was kinda refreshing, as frustrating as it was. I don't remember the last time I've ever had a challenging legendary battle aside from like... Mewtwo. Most legendaries you can wall fairly easily, but Ultra Necrozma was my first legitimate challenging, which is refreshing to see!

Although unfortunately I think I've had to sacrifice a good few of my team members to bring it down. But eventually, I did.

Hello friends, I just finished battling Necrozma (this is my first seriously played game since maybe X and Y, ORAS was boring as heck), and I thought it was nice to have something so difficult. It forced me to use Pokemon that I normally wouldn't have used (level 54 +spd Return Lopunny killed it after thunder punch paralyzed it because a monotype dragon team at level 50 is actually not that great). ORAS was bland because the legendary battles were all easy, and the same was the case with X and Y. The original Sun/Moon were also only okay with battle difficulty. This game was good. I'm glad the directors stopped moving towards making the game easy for kids to forcing you to actually plan moves and pokemon or strategize your compositions and whatnot.

I just used Mimikyu's Z-move and then finished with my Primarina's moonblast :/
It surprisingly wasn't that hard

On my second playthrough, I built up a Pokemon just for beating Necrozma: Malamar with topsy-turvy. Should've kept Foul Play on it, now that I think about it. Even with all that preparation from literally the first few hours of the game, I lost horribly until I buffed my Pokemon all the way with Refresh.

Once I hit it with topsy-turvy, it went down in one or two hits.

This isn't even the worst battle in the game. That would be the postgame battle where it's 2v1 and you're constantly being put to sleep. Luckily, I planned for that, too.

I don't remember how I beat it exactly. Each of my team members were wiped out with a single blow the first few tries, but somehow I managed a way to beat it.

But that was then. My concern right now is how I'll beat it with my current (mono grass) team...

I was also quite lucky in this regard. Having spent the my first few hours hunting for a shiny larvitar in diggletts cave, I had T-Tar (lvl 57) on hand with crunch and darkinium z to bring the pain on Ultra Necrozma.

I highly recommend using roto boost! I used two on T-Tar before using the Z move and that OHKO'd the dragon of light.

Long story short, use roto powers and try your best to be prepared ahead of time

Being prepared ahead of time may be tough if it's your first time playing the game. A lot of us were caught out by the sudden level spike when the game normally expects high 40s to low 50s at this point, and also that there are only a certain amount of mons you can pick up that will actually lay the hurt on Ultra Necrozma, such as your aforementioned Tyranitar, and Zoroark. Steels are also good because they resist all of his moves, but even if you know what's coming the problem is that it basically forces you to waste a teamslot just for it, putting a strain on teambuilding in a game where anything could otherwise work.