In order to reach the optional bosses - Legion and Nuculais - you first must obtain the fifth Innocent Devil (Devil Type). I got my bird ID from LVL1 Crow to LVL25 Crimson in less than half an hour, and made over 10k. The Soldier Skeletons drop Steel which can be forged into Lamellar and sold for decent gold. The room can be respawned by zoning upstairs and then back down immediately. Each of these are very easily killed and drop tons of evolution crystals. In Mortvia Aqueducts there is a room very early on, right before changing floors, that has several Soldier Skeletons that spawn in it.
