Stretch your Line Infantry to be as shallow as possible and set them on kneel fire. Your riflemen will be a good distance forward with your ranged cavalry (revolver/carbine is the best choice) behind them. Arrange your artillery behind your line in a well suited position. It doesn't need to be perfect but if it can attack the the portion your riflemen are engaging then it is ideal. Station your general behind your artillery for ganbattle and morale cooldown bonuses. They really tweak cannon accuracy in your favor. Let the enemy march to you.
Charge your ranged cavalry outward and toward the enemies cavalry on the right flank. Focus on killing high value targets with your artillery like the cavalry on the left flank or the units approaching your riflemen.
Once your cavalry reaches the enemies, micro manage so one unit retreats while two units fire to ensure minimal casualties. If you can't micromanage three units, try playing DOTA instead.
Your riflemen will open fire on the right flank as your carbine/revolver cavalry flank them and barrage them from the side. This creates a huge drop in morale for the enemy flank. If your artillery has not inflicted enough casualties then you will have to micro your riflemen backwards, one unit at a time. As long as your cavalry has kept up fire then they will break before or the moment when they reach your main line.
Your general should still be pumping bonus cooldowns to your artillery. As the enemy cav approaches your left flank, peel him away to shoot over the heads of your infantry and if need be, charge into the fray. Once this is taking care of itself, turn your cavalry and riflemen 90 degrees towards the center of the line. You will push upwards (towards enemy artillery) and towards the enemy with your right flank, enveloping and annihilating your way like a hungry caterpillar.
Peel off line infantry to join your riflemen on your new line. Don't be afraid to push your riflemen ahead and behind the enemy lines. Send one of your three cav units to take out the enemy artillery while the remaining two units expend the last of their ammunition and ward off the enemy general. Continue pushing towards the enemies left flank while holding steady on your own main line.
Nom nom nom nom nom nom nom. Your general should assist in taking out the melee infantry on the left flank as he can fire over the line infantries heads. Don't throw him into the fray unless you feel like losing a lot of bodyguards. Deal with the enemy general wherever he charges. At this point you'll have far too many troops for it to be effective. Keep eating the rest of the line and hit end battle after the victory screen comes up.