The Brandy Station battle is fought with other CSA units, not your own. You will be outnumbered, but the Union forces are not as aggressive as they could be, and your units despite their small sizes can rack up kills at high rates, thus routing attackers pretty easily.
You want to bring your units in to the fortified area, dismount your cav, keep your artillery units in a central place where they can fire together at (and rout) anything, and occupy trenches and breastworks without getting overextended. Your three-star infantry units are the most powerful and should man the main N and S-facing trenches. The most important sectors are N, W, and S, although the Federals will at least try to approach you from the east.
Mostly this mission runs on autopilot: Union units move in, are killed at high rates, rout, and retreat—rinse and repeat. But you really are short on units and the AI will try to set you up for being flanked, so you’ll need to do some micro to ensure that key units are pointing the right way when attacks come in. Once you get the hang of it you can run the timer down on fast speed.