After completing the first mission in Mount Steep, there will be various quests you can do. Look for "Exposition Al" and talk to the Cactus. After completing the quest, the achievement should unlock.
*** Spoiler - click to reveal ***
After talking to the Cactus, you are trying to find the location of the Instructional Podcast for your other quest item, and she know the story behind it (spoiler: she was the one who hid the podcast). You will find out that you need to complete a treasure map to find it.
Turns out, zombies have been peeping over your conversation and decide to ruin your plans. They took the map and distribute the pieces. Your job is to find the map pieces.
The map pieces will be inside marked locations. Inside those locations, there is a Champion Zombie you have to defeat. After getting all 6 pieces, the Cactus will repair the map, and due to her horrible handwriting, she will point you to the location.
Follow the points and find X's marks the spot. No surprise, you have to beat a Mega Gargatuian boss. After defeating the boss, the mission is over, and the achievement should unlock!