This achievement is for a side mission you can complete once you have completed story mission #8: The Son of a Witch. Minor spoilers below.
The name of the quest may give a hint that it is related to The Witcher, and it is most definitely that and a parody to boot, and is well done.
You can pick up the side mission in the Overworld in The Sweetsands area, and it will take you to Sunfang Oasis. Sunfang Oasis is a fairly small map all things considered, as it only has 15 Lucky Dice, and seems to have been built around this mission as there are no other side missions that take place here. Follow the quest marker to the mysterious chest, talk to the person there, and after being attacked, you'll find out he's Gerritt of Trivia. He will have you complete several tasks to free Salissa's Form, and you will be in combat with the Coiled in just about all of them. In the end, once you complete all the tasks and free Salissa's Form, you will then have to kill her to "fulfill your destiny". You will receive the Tidesorrow, Lament of the Seas as a quest reward. You then go back to talk to the Archmages, where the quest completes and the achievement should pop.
But, as noted in some of the comments in the Forums for Wonderlands, there can be issues with this achievement unlocking properly, both in single player and in co-op. I did not personally have any issues (solo).
UPDATE: According to patch v1.0.2.0b, this achievement should now be obtainable in co-op. Patch notes at
https://playwonderlands.2k.com/news/tiny-tinas-wonderlands-n.... See the Change Notes section, second bullet.