Here is a good info page of the Toto-Rak dungeon. You need 2-4 level 25+ players to enter (definitely 4 people if you want to complete the dungeon) and the grand company quest Imperial Devices open.

There's also an open world dungeon (so no entry requirements, you can just walk in) called Sphoshae (or something like that) close to Limsa Lominsa (can see it on your map when you exit the city). The monsters inside that one are level 15+.