The EVs on Necrozma let it outspeed scarf base 95s at +2 and lets it almost always survive life orb Yveltal’s dark pulse and OHKO with a +2 Sunsteel Strike. Scarf Kartana is an excellent revenge killer and late-game cleaner. Celesteela is a physically defensive pivot/wall. Magearna is very specially bulky and has a slow volt switch, which is why I run min speed to have an even slower volt switch. Ice beam on Magearna OHKOes stuff that this team has trouble with such as Lando-T and Zygarde. Air balloon Heatran checks fire types which threaten Celesteela and Magearna, and is a great check to Pdon and opposing Necrozma-DM. Specs Latios has a ridiculously powerful Draco meteor which punches holes in most teams, being able to OHKO defensive Lando-T and some Pdon sets.
Weak matchups:
Ho-oh/Reshiram - lots of strong fire types which can be difficult to check if heatran goes down
Pdon/Zygarde-C - pretty much any team with Zygarde on it is threatening. If it gets set up it’s nearly impossible to take down.
Arceus-Normal - Ditto stops Necrozma sweeps completely, and Z-Conversion Porygon-Z with Shadow Ball destroys everything except Magearna