This is story-related so it can't be missed, but since achievements are not flagged yet, I thought I'd post this guide to help anyone who might need help knowing what each achievement is.
The achievement pops after you complete the quest called "Shelter Skelter" in Plankerton, which comes right after Plankerton Storm Shield Defense 4. The quest itself asks you to successfully complete an Evacuate the Shelter quest in a 34+ area. An Evacuate the Shelter quest is a quest where you have to protect a shelter that has very little health from 2 husk waves.
The achievement pops at the rewards screen, as usual.