In order to reach the optional bosses -- Legion and Nuculais -- you first must obtain the fifth Innocent Devil (Devil Type). With this, head back to Garibaldi Temple and slide under the crack in the wall back near the area with the four courtyards and Fencers. Proceed through the hallways, avoiding the Blood Bones and eventually you'll be in a hallway that is living. Save before you enter the boss area and make sure you're at least at level 50 or expect a tough fight.