What I do (assuming you may not have read the fine print in the manual and discovered this yet) is combine units when I can. To do that, for instance using a Gondor Soldier and a Gondor Archer, click one and move your mouse over the other. You'll see the curser change to something like >>><<<. Click again and the two combine. The archers will snipe while the footmen now protect them.
This works with Rohan's Yeomen archers and Peasants too, but make sure you click the "arm all peasants" button. You can combine foot with Rohhirim horse, but that doesn't make sense to me. My favorite combination is Gondor Tower Guards, and Rangers.
Next, to every unit you have (for a mass attack for instance, press Q. To slect all horses, shift-click or doubleclick the type of unit you want to select. Then assign them a combat group by pressing CTRL-1through9.
I like to build up a base and let them come to me. My archers and foot will slaughter them, while I send my Horsemen our and around to assault their base.
To destroy a base, I generally first destroy the buildings that produce troops, then the citadel. Without the citadel the enemy will not build new or repair old buildings. Sometimes he'll rebuild the citadel, but you can bankrupt him quickly doing this.
I think back away and heal up, then raid his base again.