It depends on the helper squads for me. With a few helpers, e.g., Hungry Hunter, Siloya and Kagami I can consistently do 3.5-3.9 million damage on a 20 Necktie attack so I can clear the boss with 3x20. Other helpers I am getting 3-3.5 million and so I try and make sure I get two whacks in with one of those three helpers, or a non-ally helper with around 90K+ total power in their helper squad.