During the fight with Zagi on the Heracles you will see Zagi apply poison to himself at about 50% health. Green bubbles will pop above his head, just like if you had been poisoned.
Simply enter the boss battle directly controlling Karol or use the
Moon Selector (synthesis item) to swap to Karol when Zagis health reaches 50%.
Then get up close to Zagi and use Karols
Nice Recovery Smash Arte on Zagi when he poisons himself. That's it.
You will see
Great!!! on your screen when you do and a quip from Zagi saying that you're even now. Really easy achievement to get.