Depends on the play style. In general you should always use Primal Groudon and an Arceus form specially if you are a beginner.
You need also some hazards removal way and a strong Stealth Rock inducer. The other team slots should be used to cover the weaknesses of S and A rank mons of
the viability ranking threat either by having checks or by offensive pressure(would be good if you can cover others to from B and downward too).
Now if you seek a more specific response you need to tell me the play style(Offense,bulky offense,balance and stall) so i could be able to give you common cores too.