This is a story based achievement and cannot be missed.
*** Spoiler - click to reveal ***
After you have trick-or-treated every home in the bayou and the French Quarter you can take the boat to an island to progress the story. Here you will find a camp site and in talking with those at the camp it will trigger the battle with Kronoculus and his Kronys. Once you successfully defeat them the achievement will pop.
Focus on the clock first and then go after Kronoculus and his Kronys as needed. The battle will end when Kronoculus dies even if any of his Kronys are still alive so you can use that as part of your strategy as to who to focus your attacks on.