The Nightmare is un-replayable. Once you clear it, it's gone forever. The Concealed Ruins have Nidoqueens who like to spam the Earth Power move, wich is really cheap. Bring a Pokemon resistant to Earth-type moves or another room-wide move to counter them. The Final Maze is not the hardest of dungeons, but it's clearly one of the best dungeons that puts the player's skills as an explorer to the test without being overly cheap, on both factions.
It's not very hard, but try completing the FM with an Average leveled team. The satisfaction you get is well worth the time :D.