2 methods:

Method 1: The Waifu Method: Do I really need to explain? ;p

Method 2: "Max Gameplay Impact": Defense goes to Counter girls like Anemone (who also has Team Defense Up to make it even more effective) or HP Drain (lasts longer and therefore gets more opportunities to Drain, plus the Drain is "bigger" in the sense that more mitigation = more attacks required to take away the regained HP). Attack goes to either HP Drain or Hit-All skill girls. HP goes to Counter, HP Drain or Dodge girls (Counter to last longer to counter more, HP Drain to last longer to drain more HP + minimize wasted HP when drain triggers early, Dodge to have even longer lifespan)