You could always just use Lunala, it's quite hard to OHKO due to Shadow Shield. Oh, and, Prism Armor reduces it to a 3x weakness. Of course, this still isn't very great, as it gets OHKOed by Marshadow's Spectral Thief with its standard 252 SpA / 4 SpD / 252 Spe with Timid set.
If you have your mind set on that strategy, then let's go.
If it's just going to be like one turn, it won't really matter much. On the other hand, Necrozma-DM will likely last longer than Necrozma-DW if you choose to hold off on the Z-move. For that reason, I think this is a good idea.
While it would benefit more (in my opinion) from a Weakness Policy (due to Prism Armor decreasing the power of super effective moves), Ultranecrozium Z isn't a bad option at all.
As far as the moveset goes, you're, for obvious reasons, going to start with Photon Geyser. Timid would be the preferred Nature, as well as 252 Special Attack and Speed. I, personally, prefer an Ultra Necrozma with 4 Attacks due to Neuroforce boosting super effective attacks (with 4 moves of different types, it will more likely activate).
Necrozma-Dusk-Mane @ Ultranecrozium Z
Ability: Prism Armor
EVs: 252 SpA / 4 SpD / 252 Spe
IVs: 0 Atk
- Photon Geyser
- Flash Cannon / Dragon Pulse
- Heat Wave
- Power Gem / Earth Power
While not the most suggested set, I feel like it could work well. Photon Geyser is for Light That Burns The Sky as well as a strong STAB option. Flash Cannon is STAB before it Ultra Bursts, while Dragon Pulse is STAB after it Ultra Bursts (I prefer Flash Cannon, myself, since it provides more coverage than Dragon Pulse). Heat Wave covers Steel types such as other Necrozma-DM's. The last slot covers Fire types, choose what you'd like depending on the other types they cover.
So, I think that its a good idea to use a Special Necrozma-DM for Ultra Bursting.
Hope this helps! :)