Maybe you should build a small army at first. I conquered Pagan and Patna at first, then I conquered Chengdu and finally Chang'an. However, I played an older version and in the new versions I think civs build more units. Going for India it's a good alternative until the Mughal spawn, but I think the best strategy is to conquer the Chinese. Try to conquer Changan, Nanjing, Kaifeng and other cities inside their core area and raze some if you can't keep them. If you can't conquer them straightforward pillage the countryside (especially the irons and the horses), the mines, destroy routes, so that they cannot transfer units, try to bring allies in the war. If they are too strong consider building siege units which cause collateral damage. In my game I conquered and lost cities some times before they finally collapsed. If you weaken then enough they will be destroyed by the mongols (don't let them to take the chinese cities, orelse they will be a constant danger).