I also had a Malamar in my team so it was fairly handy for this battle. I had been spoiled on its difficulty by the internet (and also what with doing Daily articles about the game) on it being rather hard, so I had some idea about it (plus I didn't play the game that quickly...).
I don't remember how I beat it the first time but my last play through of USUM was a Castform solo and that was rather rough. I ended up having to resort to Roto Boosts and a Z-Shadow Ball to defeat it.
I remember quite back far when facing Necrozma. I tried many things such as outspeed it with quick claw, but it swipes my team. until I came a way to handle it: I use Gengar as first line up, then start using Destiny Bond soon as it 1-shot my gengar, necrozoma is good as gone. It pretty much handling its weakness to almost every team you setup expect a destiny bond gengar which can easily ko him if you are lucky to use it first.
Hmm, I used a dark type Pokemon. Realistically, there's no easy way to beat Ultra Necrozma without one.... unless you use Rotom Boosts, which I didn't for this battle.
Ghost Mono worked fine for me.
was really annoyed that I didn't realize that Ultra Necroz packed Smart Strike which oneshot my Mimikyu. I also forgot that it was part Dragon so I could have 2hkod it with Play Rough but I wasted a turn swords dancing. Gengar/Decidueye put it into range for mimikyu to be revived and have the last laugh, however.