As others have stated, attempt to go around. However, sometimes it isn't really possible (especially given the East is so supply rich... I'm looking at you Richmond), so if you are going to attack, you need to attack with a lot. Given that the game uses power ratings when rolling for retreat, try to have a power rating at least 3 times higher that the folks you are attacking. That will often cause the defending side to retreat. Also remember that MTSG stacks do not benefit from entrenchments, so if you hit then enemy on the end of the line, you can minimize the number of enemy elements benefiting from the entrenchments. Finally, don't attack with poor leadership. Have corps commanders and a CiC with high attack ratings for the frontage bonus and firing bonus.