Grathan has a lot of hype and was supposedly a parallel for Dark Councillors, based on his title of "the thirteenth Dark Councillor", except it's a self-declared title from what I recall, and he got beaten by an early Warrior. He's really not that impressive.
Skotia is implied to be more powerful than Zash and destroyed his Trandoshan bodyguards effortlessly - a similar-level Inquisitor required the anti-cybernetic device to win.
Skotia should win.