Assassin's Sangfroid could be easily dealt with the moment its aggro formula was made known to the players. A rudimentary method to deal with this would be to have the target lie flat on the ground and have the other players pile up on top of him to form a dog pile... Now that the Shadowmist Assassin was unable to attack the targeted player within, and the others were not its target of aggro, which would not shift either, would that not mean that the players could have free reign and slowly whittle it to death?
Of course, not.
That was precisely the condition to cause the Shadowmist Assassin to enrage. The moment the Shadowmist Assassin was unable to land its attack on the target, it would then become enraged.