What's a good core with Stored Power Necrozma? And what are ways to support said core?
Just a heads up, since the meta is relatively new, pokemon i mention as partners might not necessarily be viable in the long run, but i feel this is an ok step in the right direction.
So, since stored power necrozma uses only psychic type attacks, pokemon that can take care of dark types are the first that come to mind. Usually, bulkier fighting or fairy pokemon such as hitmontop, hariyama and granbull are better for the job because they can also switch in on dark type attacks relatively comfortably.
Another approach is to use a different psychic type that also lures out dark or steel types and weakens them so necrozma could sweep later on.
The remaining part of your team depends on which route you choose. if you chose something frailer, then a backup check to dark/ghost/steel types is also needed. bulkier teams could benefit from a heal bell user seeing as necrozma is very vulnerable to being toxiced.
some kind of hazard control, especially getting rid of tspikes, is very important. necrozma doesnt like being statused and if rocks/spikes are on the field, its job of pivoting and scaring out fighting types early game becomes more difficult.
this is everything that comes to mind when teambuilding to support necrozma, and the continuation largely depends on pokemon chosen before.
hope my answer helps you with building and have fun playing sm nu.