At some point it comes down to gear and the way certain skills worked at a certain time. Lashes, I think, was toned down after Ravon, for instance. Thief waylay was changed after I played Kasir. Poisoner stuff was changed after Yanacek. The standards for giving out imm exp were changed after the Mizfara/Woldrun era.
Bottom line- Gzurweeg didn't have access to as much Hell/Silent gear as Cabdru. In real game-play, i.e. not some sort of silly mano-a-mano duel, I'm thinking Gzurweeg would probably eventually come out on top, mainly because he would be more willing to employ allies against Cabdru. Scion is a smaller cabal to begin with, and Nep is a little bit more of a loner.
Most likely they'd both be trying to put the other to sleep, then bailing if that failed.