Hi, I'm teaching the Tacitus set text this year for the 1st year students of the new reformed specification. What I find works best with my students to translate the text is to break it down into smaller chunks which they translate roughly for homework. We then go through a better translation in class and look at stylistic points. About once every half term, we have a short test on a section done in that half term which asks them to (a) translate a bit of the chunk and (b) comment on the rest of it.
Unfortunately, it really is a case of just learning the text really well, both in terms of knowing your translation and also the stylistic elements, since that's where most of the marks are to be found.
There are a few resources which I can recommend if anyone's interested, so by all means PM me and I'll see what I can find.