Small example of how you can use arty (as 1 of more uses); in my battle against the greater demon of Tzeentch and his army, my cannon was shooting at the greater demon. Due to knock backs he came way later to my battle line than his cavalry. In this case I could deal with the cavalry and chariots first. The greater demon was almost alone in the 2nd wave, while the main bulk of infantry came as a 3rd (who are easier to counter-charge). This meant I could tie up certain powerfull units (used some slayers on 2 giants), while breaking others with flanking charges. My gyrocopters killed off the hellcannons and tied up some stragglers. In this way, I always had superior force against part of his army, and with some micro managing, to keep battlelines intact, it was relatively easy.