one hour or so by taxi from Xian airport to Terra cotta Warriors. I think you can stay in the city center, for example Bell Tower Hotel or melody Hotel. For there is airport bus to the airport in front of Meloday hotel, close to the bell tower. The terra cotta will not take all day lone and you can see the city at night. And also,the earliest shuttle bus to the airport leaves at 6am. Time is enough for you. There are many taxis beside the shuttle bus which run to the airport at any time.

Thank You for your reply. Do you recommend I hire a tour guide?

If you only go to visit the Terra Cotta warriors, do not have to hire a guide. There is guide in the museum of terra cotta. You can have one if you need. Negotiate with the taxi fare before you getting on the taxi. After that, told the driver to take you back to 钟楼 （Bell Tower） or Melody Hotel （美仑酒店）nearby.