Overkill occurs when the player deals a lot of damage when defeating a MAD.
To activate, deal damage equal to the enemy's remaining HP or its maximum HP. It's easier to activate if you still for a damage amount over the limit of an enemy's HP. Upon activating Overkill, the Verse Maiden who caused it will get a Bonus added to her Action Count, and Takt's Action Count will recover. Also, you can obtain many bonuses such as an added Result Bonus in addition to dropped items exclusive to Overkill.