Really, guys, the only thing that works on Mazoku Lords in Slayers is the Giga Slave.
That is canon.
Gaav, Hellmaster BOTH showed that they could shrug off the Ra Tilt, and the Dragon Slave. (Granted Hellmaster in the novels got hit with two Ra Tilts and a Dragon Slave and that nearly killed him, even he admitted that.) Hell, Gaav got hit with the Ragna Blade, got cut, and GOT BACK UP!
Hellmaster even said "Don't try the Ragna Blade, I'll just teleport away. Those things hurt like a bitch." Which leads me to think that he's been hit with them in the past and survived.
The idea here is, Nanoha, after studying Slayers-style Black Magic, and black magic from other universes (they exist), came up with a Black Magic variation of her own magic, so that she could deal with the likes of Hellmaster, Gaav, Shabby in case she ran into them ever again.
It just has a VERY bad side-effect of fucking up her body when she uses it.