As you can see, from the starting area you have to go DOWN to the main floors. From there you can choose to go down to the lower levels or up to the B or A towers.
The lower floors are generally easier to traverse since the monsters aren't aggro and only Umbrellas and Twinkles will attack if they see you cast. Hell Divers and Platinums are the generally trained on mobs.
Upper floors can get a bit complicated due to the A B split, but the map is relatively informative. Generally the layout of a single room is like this:
Red dots are the entrances from LOWER floors, while blue dots are stairs to UPPER floors. The little doorway between the 2 blue dots is the passageway from A8F to B8F shown in the schematics. This is ONLY on A8F and B8F and not found in any other floors. Sometimes there will be walls between the red and blue dots.
Usually people would train at B7F, there are Hell Divers, Fox Trot B3s and Rameless, which are all passive monsters (Rameless seem to attack any other casters around it after the first attack).
For ectoplasma campers, they can be found at A-10F and the connecting passage. If you don't want Kodamas raeping you all the time you'd train at the connecting passage.
Credits:ec.gamedb (link here) and eco.gametsg (link here) for knowing their stuff. Also more thanks to eco.gametsg for making the map so I wouldn't need to make it myself \o/