He has won 8 or 9 battles against Epiros now and the last battle was 900 Romans vs 1500 Epirots. They outnumbered his forces by over 600 men and not only did I win the battle but I killed all but about 100 of the enemy soldiers. He has the Good Commander, Strategic Expert, Hates Smelly Greeks and several other traits that give him combat stars and morale bonuses but not Vanquisher. In the past I have seen it come up after just one fight against Epiros but that was on Medium game difficulty and I am playing on Very Hard right now. I guess it's just random luck then. I guess I can just save before a battle and keep doing it until he gets the trait.