Well depends who you are and how many he can field. Before 1550 his tech group has a better army so your best bet is to have more troops then he has. After that you need about 100k as Western European. As persia in the mountains you need about 30-60k in the mountains. There you fight defensive until his manpower runs out then slowly advance. As Western you can call allies into the war beeline for straits try to blockade them and move in 25 k stacks close to each other. Beware of 3 star monster stacks.