I'll go ahead and say that I had surprisingly good luck with the “run like heck” strategy. You might get lucky after a try or two and slip past the Union line and get to the barn. You should probably just fight them though. Check the map and see how many survivors there are. If there are a lot, stick to the road. There's more cover and it's easier to escape. Engage the outliers with your Henry rifle and then pick up a sniper rifle if you can.

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In a minute, a huge Confederate cavalry charge will rush in and kill a lot of the Union soldiers on the road. They get blown apart by the Union artillery though. Unfortunately, a lot more Union soldiers will then spawn all around you. You need to fall back. Go all the way back to the first rock were you killed the officer. This should trigger a massive Confederate counterattack, which will soften up the Union forces. Advance along the upper road again and help them clear the path. Move carefully and focus on surviving the battle. Once you make it to Dexter by the cliff, you're almost home free. Go up to the barn and fire on the Union soldiers within. Take out the sharpshooter on the roof first, and then fire quickly at the group of 10 inside. Round the corner and finish off the last few men guarding the box of ammo.
Once they're dead, look down and keep hitting “e” to pick up a ton of ammo for the gun you've been unofficially carrying. The AMP 60 is one awesome weapon. It can auto-aim at targets to the point that it basically does the killing for you. You can carry 1400 bullets in reserve, so go ahead and fill up. You'll need them to turn the tide of Toomb's battle.