Necrozma COULD attack via telekinesis. But, one of the resistances from Superman's profile:
Telekinesis (
Forced his way through telekinesis)
It could try to Dimensional BFR via Ultra Wormhole. But forcing Superman through it might be difficult, & could it keep the portal open during a fight? What about leading Superman into the portal?
It isn't really self-BFR if it can return immediately (Much quicker than in a week.) so it COULD open portals and assault Superman, then go to another dimension, possibly to goad him into following, then close it off, leaving him stranded and alone; AFAIK, Superman doesn't have Dimensional Travel of his own.
But AFAIK, such a strategy isn't In-Character for Necrozma.
Could Post-Crisis Superman live hundreds of years? Is outliving even a viable strategy? (Especially since some folks would call retreat without return a loss.)
Confusion, a telekinetic attack, could also inflict Confusion on Superman. It could intensify Gravity, but I'd bet Superman would resist it.
It has a myriad of energy attacks. How quickly can Superman absorb them? Instantly? Would they all fail to damage him?
Supposing so, it could try attacking up close with Metal Claw (Which 1 in 10 times, raises Necrozma's physical AP by about 50% of its un-modified AP for that form.), Slash, Night Slash.... It could achieve a bit of range with Rock Blast, Stealth Rock....
The closer to maximum stamina the victim is, the more damaging Wring Out is.
It could also augment its physical durability with Iron Defense, as well as its Speed (& make itself 100 KG lighter) with Autotomize.
It could also heal with Morning Sun &/or Moonlight.
The problem is, even if it went full "no energy attacks" with the above options, there's the issue of Stamina:
Stamina: Very high. Extremely high as Ultra Necrozma.
Stamina: Essentially infinite as long as he's under yellow or blue sunlight.
Without knowing about this, Necrozma's best chance would be to absorb ALL of Earth's sun, to deprive of Superman of it. However, it might actually try that if it gets desperate.
But could Necrozma do anything to stop Superman absorbing the yellow sunlight back? Could it convert the yellow sun radiation into a different type of energy that doesn't aid Superman's stamina. Could it absorb the whole Sun fast enough before he stops it?
Worse yet, how many sources of "yellow or blue sunlight" WOULD be near Earth? Necrozma wouldn't know they fuel Superman, but it would have to get rid of all of them if it wants to exhaust him, and as a bonus, they'd power it up.
....It'd just have to not use its energy attacks and stick to using its telekinesis (Hopefully inflicting Confusion.), claws, Iron Defense, Autotomize & healing.
So yeah. It'll be a dark day on Earth when Necrozma wins. Literally. Specifically, because to exhaust Superman, it seems it needs to extinguish all the sources of yellow &/or blue sunlight available. Which, considering it can provide/steal light to other worlds, & is 4-B for illuminating all of Ultra Space, it MIGHT be able to do.
Less likely, it Dimensional BFRs Superman into an Ultra Wormhole, by leading him in or forcing him in, &/or using them to take cheap shots at Supes, possibly goading him in.