The Valkyrie soul from can actually linger long enough for you to get a free double kill, on anything you used it on. You must have killed the enemy in one hit, and you must be near an exit. Use Valkyrie (Up+B), and then quickly jump and exit the room. Turn right back around and re-enter the room. You should still see the Valkyrie there for about half a second, and anything touching its sword dies (provided you're strong enough).
Submitted by: Kenjoki Ikari