Shposhae:
Recommended Level 15+, located just East of Limsa Lominsa.
I'd say bring a party of 4 or more. A healer, someone who can take damage, and someone who can dish it out. You might want 8 people if you plan on going to the very depths. There are multiple floors that do not necessarily connect to each other, giving you lots to explore. Inside you will find 3 NMs, one of which is the Coeurl, a famous FF monster. He has a special trigger to make him pop that involves killing all the Wolves on the lowest floor.

Inside you will find Keys scattered throughout the floors, and chests that the keys unlock. Some keys drop only from the NMs, others drop from the monsters found inside Shposhae.

If is a great place to amass EXP in groups at that rank, even if you don't want to get drops. This is similar to how the Strongholds are excellent EXP sources for 40+