Gate to the Netherworld Achievement in Octopath Traveler II

This achievement is for defeating Galdera, the superboss of this game. To initiate this fight, you first have to complete the story, including all crossed path story segments, and a few specific side-quests. This should be the last thing you do in the game anyway. From here, I'd rather point to some resources that have already laid out this fight in detail rather than regurgitate it in a slightly new way. The video down below shows a "cheese" method that would be your best option. You could win the fight with characters in their 40's or 50's following that video, as well as the guides HERE and HERE.

For an even easier time, feel free to over-level and get all your characters in their 70's and 80's using the methods HERE. I used the dancer method to get everyone to the 40's, then completed Partitio's Chapter 4 so I could hire Alrond and use that method instead. With 1-2 100x XP drops after killing a Cait enemy, you'll shoot up in no time.