Try to get you chinese visa at home and not at the embassy in monngolia, because the require a tickt to china and a ticket out of china. We were planning to just enter China and travel overland to finally take a bus to Laos, but bus tickets are not accepted (plus im not sure if it's even possible to get a bus ticket e-mailed to you). Another option would be to just show them the ticket into china; then they will give you a transit visa for 15 days and you would have to extend the visa once in beijing, which still requires a ticket out of china, but at least you could reserve a train ticket and then pick it up in China, since no company will deliver a chinese train ticket in Mongolia. So be prepared!