i did this assignment aswell and you really need a lot of analysis of sources and a strong argument with a clear thesis.
i would start by just reading about the whole topic and then picking just one aspect of the fall of the dynasty and this will allw you to analyse sources about thhis aspect in a very detailed manner.
like if you focused on how one person/event/whatever effected the fall of the dynasty.
trust me, if your focus question is too broad you pay for it with a never ending, rambling essay and thats never good :|
hope that helps