What bus do i take and from which part of xian city wall to Terra-Cotta Warriors?

There is no direct bus from the City Wall to the destination. You are suggested to finish the city wall tour from the North Gate. The North Gate is the nearest one to the Main Railway Station. Then take a taxi to get to railway Station.
You can find several direct tourists bus to Terra Cotta Warriors at the East Square of the railway station. The buses are from 07:00 to 19:00 with an interval of about 15 minutes. The duration is about 70 mins. It costs CNY 7 per person.