Finding the Soul of Steel
The Soul of Steel is needed in order to clear several obstacles in Mission 5 of Devil May Cry 3: Dante's Awakening. It can be found in a room at the top of the stairs near a group of Hell Pride enemies. Once inside the room, you can use Vajura on the mechanism in order to lift the cage off the stairs. Here you'll find the Soul of Steel, which is not only required to complete the mission but also is required to access the secret mission that can be found in this level.
Secret Mission No. 2
Secret mission No. 2 can be accessed during Mission 5. To do so, you'll need to use the Soul of Steel on the big door after collecting the blue orb fragment. After entering the door, press Circle near the box on the wall near the rune clock.
The object of secret mission No. 2 is to defeat all of the enigmas without being damaged. The easiest way to do this is to jump continually to avoid their blades, attacking whenever you have the opportunity. Finishing secret mission No. 2 yields another blue orb fragment.
Boss Strategy
The boss fight at the end of Mission 5 consists of two twin fighters: Agni and Rudra. They employ a variety of sword attacks and can be quite challenging without the correct strategy. The easiest way to defeat these two bosses is by striking them immediately after they attack. Doing so will knock the sword out of their hands, which will allow you to attack again and inflict the maximum amount of damage.
Another good strategy to defeating Agni and Rudra is to reduce the health of both fighters significantly before finishing them off. Once the first fighter is defeated, the remaining fighter will use a new set of sword attacks that are even more powerful than before. You can reduce the amount of time you'll have to withstand these stronger attacks by making sure both twins are heavily damaged before finishing either of them off for good.