Actually, that scenario can be represented in game as well:
Blackbelt uses Defensive fighting (-20% ws to opponents) & the combat master talent (no bonuses for outnumbering), any hits they may land (remember, average skilled human combatant only has a 30% ws), he can parry, and even counter attack (a blackbelt and the years of training it takes would easily have a higher ws, possibly as good as 50%, assuming he's at the pinacle of his martial arts training). If the attackers are unarmed, even above average combatants would be nearly unable to land a hit (remember that unarmed attacks net you a -20% to hit against an armed opponent). The attackers can all out attack if they want, to try to make up for the defensive fighting penalty, but they still will have trouble hitting, and if he parries and counterattacks, will have no ability to dodge or parry themselves. If the attackers get lucky, and all hit, well that is simply a statistical aberration, and such luck can happen to even the most skilled opponents.