I'm currently leveling Dahlia, and plan to replace Southern Cross with her in my Counter squad, or maybe switch the two around depending on the occasion.

My idea is, that for a boss with a Single Target special attack, Southern Cross would be better. Single Target attacks deal a lot of damage, and only 1 girl may counter to it, so Southern Cross is better to reduce those attacks and replace them with regular attacks.

However, an AOE boss attacks multiple girls, and gets a ton of damage from the counter girls in the squad. Reducing the amount of AOE attacks is not beneficial for the counter strategy at all! So, it'd be better to use Dahlia against these bosses, to reduce their damage while still allowing for as many AOEs as possible. (My counter squad can take half the health from the mid-boss in ?-8 when countering it's AOE, which is way better than I actually expected)