In ELO mode, which is the most strategic mode of the game, the vast majority of your cards should be able to serve at least two functions. That way your strategy becomes more difficult to predict. For example:
-SoA with at least 4 damage: This can be an early bluff, used defensively to prevent an opponent's powerful ability without spending pills, or used as a fury counter threat because it will go through most DR.
-SoB with high power and/or attack manip: This can be used as a "safe" opener if your opponent doesn't have any high gap cards, or as an endgame wall if you land a high-gap card of your own. Of course it can be used defensively as well, in order to stop +damage bonus clans.
-DR or Defeat card with some gap ability: This can be used early to mitigate your opponent's damage if you are confident that they're not bluffing, or saved for endgame when you can decide whether you need a DR or another hit.
Early game cards are cards with Heal or Poison, Walls, Pill Reducers and other cards that would have a great effect when played on the first round.
Mid Game cards are cards with some high power or/and damage, Pill Increasers and recoverers, those with Confidence/Revenge etc.
Late game cards are those that can reverse the battle like Life steal or Life gain. high damage cards.
Utility are those cards with SoA, SoB, DR, Stop cards and all the likes.