I played Antietam yesterday on hard as confederates 35k against 83k Union.
At the end i lost 13k while the Union had only 12k soldiers left on the field (most units shattered)
Antietam can be a real bloodbath or quite easy if you know how to play it.
Guide:
1. Make sure you field a small army because the enemy will scale like crazy.
2. Put all your largest brigades on the left flank, only a few brigades at the center and you need nothing on the right but one brigade (the game will not let you start the battle without at least one brigade there)
3. You should be able to match the enemy army size at the beginning. Because you hold superior defensive positions its easy to hold them of.
4. I advice you hold the woods north of the western hill. If you put a sniper unit on der upper left "island" they can probably kill one or both of the iron brigades without much help. Later the snipers can take out their cannons from behind, so you receive fewer losses when pushing the enemy back.
5. When your reinforcements arrive quickly push the enemy back to the upper right corner of the map, you should be able to shatter most brigades before their reinforcements arrive in this corner. Make sure your canons are positioned to fight their reinforcements in the next step.
6. Make sure you to put your brigades into the woods when their reinforcements arrive. You should be able to bottle them up and destory them to the last soldier even while they outnumber you.
7. While you wipe out the enemy in the north, position your troops in the center at the bridge. When you put at least 4 brigades and 4 artillery in firing distance of the bridge you can literally kill brigade after brigade instantly. As they spawn right before you with artillery and cavalry they probably manage to rout one or two brigades of you. Thats no problem, the enemy will falter quickly when being fired on with your artillery. But beware: if you lose this position completly they will swarm you (you can still win but will suffer additional casualties).
8. Hold this bridge until victory. You will be fired upon with artillery, so its good to position some rookies there to catch the bullets.
9. When the last phase starts your north army will have finished their job and can move down to defend Sharpsburg and the sunken road, if the enemy decides to corss the bridges in the south in force. If you want you can recapture the bridges there, but its not needed for a victory.