Though one tip. .... If you want anything that resembles a challenge when going through the Elite 4, don't use true form Necrazma. That mon sh***ts all over the Elite 4. He even comes with all the moves you will pretty much need and I've only ran into like one or 2 mons I needed to switch out due to the few mons Elite4 had that could resist his moves well enough.
Jesus that mon is OP. I serously think this mon would destroy Arceus in a 1v1 lol. This thing would be lucky not to end up in the super banned category along side mega-rayquaza.
So who would win between Ultra Necrozma and Mega Rayquaza? Rayquaza's my all-time favorite pokemon and I loved how it usurped Arceus. If I'm not mistaken, U-Necrozma's ability powers up its Super-Effective moves while Mega-Ray's Delta Stream lowers moves' powers with type advantages over Flying-type. I noticed too that sometimes the craziest and biggest (in size) looking pokemon indicate how powerful they are. Mega Ray and Arceus, Primal Groudon, but then Hoopa-Unbound looked scary and over the top, but it's deceptive because it doesn't measure up to those aforementioned mons at all. So when I see U-Necrozma, I think it's instant Ubers tier, close to Arceus level. That appearance also works with smaller mons, when I see Zeroara I just think Shiny Lucario and Zoroark (seriously those two names are too similar) had sex and shat out that. Pure Electric Mythical? That sounds like hearsay but there it is. It's nerdy to guess its tier placement, but yeah, OU all the way.
I like these USUM discussions, I get all nerdy over Pokemon, most fun I've had in this forum in a long time.