Frogbeasteggs two guides, A Beginner's Guide to Medieval Total War and The Complete Total War Unit Guide contain useful tips about how to use units in a tactical (rather than strategic sense). She gives very clear suggestions as to how to make the most of each units abilities - well worth a read if you haven't already.

The replay idea is an excellent one, but I am not sure if you can select portions of battles to replay. Wathcing an comlete battle for 20 seconds of action to illustrate a point may not be worth it.