We were just in Xi'An a couple of weeks back.

Yes, the Terracotta warriors won't take the entire day. For us, we took a taxi from our hotel (Sheraton Xi'An North) to the main Xi'An railway station. From there, we found the bus area, where they had tourist buses (at least 2 different companies) plying this route. Bus fare was inexpensive, I think 7 or 8 RMB. The conductor accepts cash. The bus, airconditioned, will bring you all the way to the Terracotta warriors (there are stops along the way, but don't get down). Everybody has seats. The ride can be as long as an hour though.

Made this video which also has some footage of the place to catch the buses. Hope this is useful.