I'm pretty sure just having an all S+ card set is enough. The spirit grants a small extra boost in your stats in pvp and pve I think and if the spirit doesn't add much survivability there's always the defense levels some of the cards add. So I'm pretty sure that the best cards are S+ unless people can find certain combos that add like 15 defense levels for the whole set or something which I doubt is likely because certain cards (ex. Longevity, destroyer, battle) don't have attributes like that and only have some such as like added physical attack or magical attack not necessarily defense levels.
