Okay, I was having a lot of trouble in Ultra Sun and Ultra Moon beating Ultra Necrozma. I mean, I was prepared to get a team of Steel-types and try my best, but I thought, "There has to be a better way." Then I found out about one.

You really don't need to bother with a single Steel-type. All you need are a Poison-type to go in the last slot of your party, and a Zoruark to lead. Your Zoruark's Illusion ability makes it appear to be a Poison-type, and so Ultra Necrozma wastes a few Photon Geysers trying to hit your Zoruark with attacks that literally can't damage it while you lower its health, and then, when it's low enough, drop the Illusion and use a Black Hole Eclipse to finish it off.

So if you're having trouble with it, give that strategy a try!

I had Malamar hold a Quick Claw so it would move first, then used hypnosis to put it through I sleep and then destroyed it with Foul Play. Worked for me!