You can level up fast by completing all the assigned tasks and challenges. Also, interacting frequently with other characters and joining clubs can help.
Do the theme dungeon quests. They are designed to give good experience for your level. For example, the Ellinel Fairy Academy dungeon has quests that can quickly boost your level.
Do a lot of missions. The story missions usually give a good amount of XP. Also, participate in races. Even if you don't win, you still get some XP for taking part.
One way is to complete quests regularly. The more quests you do, especially the ones with higher difficulty levels, the more experience points you'll get. Also, try to battle stronger monsters as they give more exp when defeated.
We were unable to find a specific method to level up A Dream to Level 71. The search results mentioned some strategies and techniques for leveling up in the Legendary World and A Dream of the Underworld, but there was no mention of the specific method to level up in A Dream of the Underworld. Therefore, we are unable to give an accurate answer.
Another way is to focus on the heist missions. They offer a large amount of RP when completed successfully. Plus, you can split the work among your crew members to make it more efficient and increase your chances of success.