I like the fact that you can do a close range combo with the DM-13. What does the "DM" stand for? Devil Masher?
Here's a boss idea...
Dark Knight of Emptiness: Shadow Gear
Shadow Gear is a humanoid demon, about as tall as Vergil (maybe a bit taller.). His signature armor is made of numerous "trophies" that he's taken from defeated opponents. He uses a large sword that is made of numerous sections, meaning it can be both a bladed whip and a long, thick sword. His shield can also turn into an automatic rifle. When you defeat him, you get both weapons. His attacks are easy to predict, as he will talk before each one. His voice is deep and menacing, like Exdeath from Dissidia: Final Fantasy.
"Fall into despair!" SG will throw you to the ground.
"Suffer!" Energy balls made of nothingness. They slightly home in.
"Can you still fight after THIS?!" A long combo that (if every slash and shot connects) can take out almost half of your health bar, if you didn't pick up any blue orbs, that is.
"Why do you continue on the path of darkness?" Energy balls made of both darkness and light. If they're shot down, they relinquish White Orbs.
Also, here's an idea for a gauntlet weapon for Vergil. This is the weapon he trained with before using Beowulf.
"Miles Militus" (Lit: "soldier")
It looks like a cross between Gilgamesh and Beowulf, with the design of Beowulf, but the red glow of Gilgamesh. Also, the face-mask is permanent for as long as it's equipped. It controls the element of Water.