Elf: Dunye, Fiona and Barry, the last boss to lead to E world.
Lei Guangyu: Claire.
Pocket mouse: Mars
Cass magician: Rhonda?
Flying Sword: Barry, E World Red Pearl, Ronda
Personally, I think the difficulty of the dungeon here is related to the distance of London. Mars and Fiona are easier to play than beginners.
These cities have no limit on the number of people or the level.