I think you can get there at lvl 50 or so. But you gotta buy Wrath of Centaur key from the herbist for 1m first :p Use that and you get 12 minutes in the cave, and you can tele there through guard captain.
When inside the cave, you can find 2 kind of monsters there; Vicious Centaurs and Vengeful Centaurs. Vicious Centaurs are pretty much worthless, so just focus on trying to find the Vengeful Centaurs
They will drop 500k and probably some material too :p Trick is you gotta find em from a bunch of Vicious Centaurs lol
As a sidenote, i can be wrong but i remember loong time ago hearing that when you have ~900 defense, the centaurs hit only 1's on you o.o