Odds are when you made it to the Altar of the Moone, you didn't expect to literally be thrown into battle with Dawn Wings Necrozma. And honestly, it might seem a little intimidating at first with being Level 50 and having had its stats bolstered with its new transformation. But don't fret, as Dawn Wings Necrozma, at least in this battle, isn't as difficult of a Pokemon to defeat.
For one, it might continuously use its move Moonlight instead of attacking for some weird reason. If this happens, just keep hitting with Super Effective Dark Type Attacks - its greatest weakness of all - and it will be down for the count before you know it. If you aren't so lucky, continue to go for its weakest point, that is Dark Type Attacks. Using the move Taunt will also prevent it from regaining its HP with Moonlight, which is nice if you have a Pokemon with such a move. Using Physical Moves are also recommend over Special Moves, as Dawn Wings Necrozma is definitely weaker in this given area.
If worst comes to worst, it will take you a few attempts to defeat Dawn Wings Necrozma. This is nothing in the grand scheme of things, as you should always save before entering the Altar of the Moone in case you do need to reset without losing any ground. The real challenge comes after defeating Necrozma in this given form.