First, find like-minded collaborators with similar writing styles and interests. Then, clearly define the story's theme and plot together. Good communication is key throughout the process.
Doing a collab story involves a lot of teamwork. You both need to respect each other's ideas and be willing to compromise. Outline the story structure beforehand and take turns writing different parts, while constantly reviewing and editing each other's work.
Well, maybe the collab introduced ideas that clashed with the original essence of your fanfiction. For example, if the other person had a very different vision of the characters or the plot. Their ideas could have forced your story in a direction you didn't like, or they might have made changes that disrupted the flow you had carefully crafted.
You could try to salvage it on your own. Go back to your original ideas and rewrite the parts that were ruined. Cut out any elements that the collab added which you don't like.
One of the great Danmachi fanfics is 'A Different Perspective'. It offers a fresh take on the characters and their relationships within the Danmachi universe. The story delves deeper into the backstories of some minor characters and makes them more significant. Another good one is 'The Unseen Adventures'. It takes the characters on new quests that are not in the original story, adding more excitement and mystery.