Well, two nights ago while I was trying to finish off the Vandals, I had an 8 subterfuge assassin near a stack that only contained the Vandal faction leader and heir. With 8 subterfuge, he had a 16% chance to take out the faction leader and a 14% chance to take out the heir, with no other troops in the stack to lower the chances. Granted, they might well have possessed traits or ancillaries that increased personal security in addition to being the faction leaders, but I didn't even try. I just killed them in battle.

On a side note, killing them in battle has driven me NUTS. On at least 3 occasions, I've managed to destroy the bodyguard unit of the faction leader of the Vandals and Huns, only to have the general escape the battlefield alone. One turn later, he's back up to 19 bodyguards. Give him a full year and his bodyguard unit is nearly full strength. In all of these cases, I didn't have a unit of toxic hippos around that could chase him down. You really need a unit or two of super fast cavalry to keep these guys from escaping.