[Nick_K]

In my experience attack rear targets the rearmost unit of regular troops. I have not seen it attack lone mages, although I haven't tried it with such small numbers. Small units can potentially rout very quickly indeed, even with heavy cavalry's good morale. I've recently seen an AI unit of two elite knights (of the chalice IIRC) break without even suffering a casualty. I'd recommend keeping them together in one group. Set as far to the front as possible. Might try to attack rearmost but they'd probably attack the troops - I'd hope they'd break the enemy troops and then run down the mages.
When you say you 'have no idea' what you'll be facing.... unless you're attacking indeps it seems unwise to enter battle with such a small force. An enemy army could easily massively outnumber yours. Against a large force you'll never close with the mages.