This is story related and cannot be missed.
Expect a lengthy span of time and effort between the initial few achievements which come quickly and this one. -Gilbert Prime
The last mission you’ll complete will be:
*** Spoiler - click to reveal ***Sowing the Seeds of Success: Part 3.
You will have finally finished the Deitree by using the Water of Life item.
You will then have the final boss battle with Brainy Badboon. Once he is defeated you’ll talk to the villagers and decide on who to bring with you to the next island.
Once you’re done fast travel to the Dock and speak to the captain. Once the short scene is done your achievement will pop.