I'll repost one of my old artillery posts:
Artillery is much more effective in support than when bombarding alone. Therefore, whenever possible, always try to add a small sacrificial attacking unit to your bombardment. That way, the artillery will be considered in support of that attacking unit by TOAW, and will cause about an order-of-magnitude more casualties.
I suppose Norm's rational for this effect was that if there were no accompanying ground attack, the defenders could disperse and hunker down much more than if they had to hold off attackers while being bombarded.
In addition, artillery bombarding alone are subject to counter-battery fire, which can be devastating. Supporting artillery are not. That's probably a shortcoming of TOAW, but players need to take advantage of it. You can be sure your opponent will.
Of course, you have to have a unit adjacent to the defending units to take advantage of this. Set its loss limit to minimize losses and the bombardment will probably only expend one round of combat. Also, note that artillery has little effect on armored equipment types. Be sure the defenders are rich in soft targets (squads, guns, transport, etc.) before setting up such an attack. And similarly, since the defenders will probably have their own supporting artillery, the best unit for the ground attack would be one with mostly armored equipment (recon, armor, etc.).
In fact, that last point is a good rule for all attacks. Try not to expose your own infantry squads to enemy supporting artillery by restricting your attacks to your armored units. Save your infantry for defense. Artillery is REALLY good supporting a defense. Of course, this isn't often possible, since armor can be in short supply, but it is what you want to try for. On the other hand, if the defenders are out of range of support, you can feel free to clobber them with your infantry.
That raises another point. It is critical to ascertain the location and status of enemy artillery units when planning your attacks. Small, preliminary attacks can cause supporting artillery to reveal itself. Recon can find them too. Note whether the artillery units are in support mode (reserve or dug-in). If they are, try to knock them out of that mode before proceeding with in-range attacks with infantry. Once retreated or in mobile mode, they are out of support mode.
This raises a final point. Remember that when your turn ends, your artillery will need to be in shape to support your defenders. To do so, they need to be either dug-in or in reserve mode. And they then need to be protected from subsequent enemy ground attacks. That means that late turn attacks should have indirect rather than direct artillery support, because giving direct support risks that the turn will end before you will be able to put them back in a support mode. Then they will be in mobile mode in the defense turn and unable to support the defenders. And it means that they should always be deployed behind able ground units to keep the attackers at arms-length. Allowing your opponent to easily reach your artillery with ground attacks is bad form.
