I got to TWTNW on level 50, and I've been training there. Just go from the beginning of the castle, till the Altar of Naught, then go to world map and start all over again. I go through all the places that have Nobodies in it. I generally level up once every trip to the castle, and I'm already on level 83. I figured, it'll take me more or less 16 more trips around the castle, then I'll be level 99.