In Ninja Gaiden II (Black)/Sigma 2, each of the 4 Greater Fiends have a counter hit state you can knock them into being. It depends on what attack they're using. With the right timing and weapon, and depending on the attack, you can interrupt, stunning them and/or taking a huge damage out their respective life bar. You can do this with a Level 1 charged Ultimate Technique. There are hidden orbs you can find in Hayabusa's Village late game (or in Chapter Challenge Chapter #2) where you aim your bow and air at paper targets. There are three of them, and each three has a group of different color orbs. One target has all yellow orbs, the second target has red orbs, and the final target has blue orbs.
On Mentor and above, certain orb chests are randomized, and you will either get an orb, ghostfish, or bats will spawn from them. Killing them quickly nets you those orbs, but they will be randomized.