How should I tackle a really long and complicated unseen passage?

Firstly identify the main clause! Find the verb, then the subject (if there is one) and then the object. Look for any particular 'signpost' words that might suggest there is a subordinate clause (e.g. 'cum', 'qui' or 'ut') and bracket these off. Annotate the passage by parsing each word and then seeing what agrees. If there is any vocab you are unsure about, see if it looks similar to any other Latin words as they might share a root or if there is an English word that is similar. Once you can see the basic structure of the sentence, solve each clause separately and then link them together to form the whole sentence. Don't try to tackle it all at once!