To get "True Millenium Tower". I was forced to grinding three times and beat Amon.
After that, you got an enterance of "True Millenium Tower".
Pro tips: Try to maxed out everything before entering this dungeons. Stocks lot of foods/energy drinks etc. Be prepared for 1k-2k or even one hit kill skill from Shin Amon.
Use Nanba (Homeless)/Eri (Dealer) as "Mage" to decrease their defense and attack, also for healers. While the rest is up to you.