I don't think Sardonyx is a "build around" card; I think he is a reactionary late-game card against large swarms of creatures. I mean, his sweeping attack rolls 10(!) dice, which is ridiculous. Not even Adramelech rolls that many. That's the main reason to use him.
For the mage that isn't summoning as many creatures and is focusing on buffing enchants instead, Sardonyx is pretty much there as a last resort in a match where you know you aren't going to win the late game. You pretty much trade your late game for the ability to have the most powerful creature in the game right now, specifically designed to hose hordes and be borderline untouchable.
I think the Arraxian Crown Warlock is probably best suited to use him, with his 38 health and ability to heal himself for the damage he takes in the early/mid game A.C. Warlock also doesn't seem as good with the Pentagram, so a last-resort option in case his late game isn't looking great is more desirable.