As the other solution states, this game is glitchy, and this achievement may not pop upon completion. From researching this in depth, this seems to be a unique problem tied to just the Xbox One version of this title, but no version is free from glitches honestly. This is what I did to try and circumvent it glitching and it seems to work. Thanks to Fireal20 in the comments of the other solution for the foundation of this!
To be on the safe side, when the game tells you to "Return to Ice Kingdom" to confront the Ice King to get more info, that's technically a point of no return. Make sure before you head there that every side quest has been completed as heading there locks you into the final boss with Gumbald unless you dashboard. Failing to do so may bork some of the side quests, mainly the one involving the seven pirate ships. Before fighting Gumbald I was only missing:
Sweet Victory (beating Gumbald)
The achievement for pulling the plug
The four achievements tied to leveling the crew to 10
The "Platinum Trophy" achievement for having every other achievement.
After fighting Gumbald I went directly to pull the plug and did so, skipped the credits, reloaded my save, and went to grind on enemies in the Fire Kingdom. The game explains pulling the plug as ending the game as it will drain the water, it does not. The game will reload in a completed state with the water back to explore. A lot of users here who have saved that achievement for last had the completion one glitch on them.
I didn't run from a single fight in the campaign and went after most chests guarded by enemies and after Gumbald only BMO was level 9. The grind for everyone in the fire Kingdom took me an extra half hour after pulling the plug, and I tried to pace the bonuses in a way that wouldn't pop two leveling achievements in the same fight. This one popped before I got the Hero Finn achievement.
TLDR:
Do everything side quest related and miscellaneous achievements (Lighting the fires, finding the snail etc) before returning to see the Ice King.
Fight Gumbald there.
Go directly to pull the plug after.
Skip credits, continue back into the world, head to Fire Kingdom to grind for XP to become level 10 with everyone.
In this order, your chances of it unlocking seem to be higher than by pulling the plug last. Good luck and let me know if this worked for you. At the time of posting a patch is APPARENTLY still in the works, but I wouldn't hold my breath.
UPDATE: This is not a be all end all guide, moreso how I managed to get the achievement to pop. Your mileage may vary!