With Astra Militarum, we have some of the best firepower around, if not the best. However it is important to use it correctly and both build your army to properly take advantage of it, and to deploy it so it can make use of it. With some of the longest ranged weapons in the game, and some of the weakest units in melee, it is important to layer your fires so that you can weaken your opponent, or attrit in military terms, before they reach your lines.
The main concept is to have the ability to hit the majority of the field, with your fire increasing as the enemy approaches. Use your direct fire weapons to cover avenues of approach that will most likely be used and to hopefully funnel your opponent into your killzones or maneuver where you want him to. Artillery is one of the first things to look at, because we have the best, and it can hit units that are hidden behind LoS and generally at long range. The Wyvern and Manticore are my personal favorites here, the first for its simple efficiency at destroying infantry, and the second for its range and utility.
Once you have indirect fire you want to bring strong direct fire units that can cover direct approaches. Leman russes are great for this as they are durable to stand out and most variants have the firepower to support themselves. When deploying you want to ensure your direct and indirect fires are linked together supporting each other. A good layout and example is below.
The manticore can reach to hit most if not all the board, with the wyverns providing mid range fires as units get closer, leman russes provide direct fire to the mane routes that most enemy forces will approach. The artillery can support the tanks and other units providing direct support and the tanks can do the same. This allows fires to be massed on targets that are high priority as well as ensure you can engage across the breadth of the field. With interlocking support, long range fires can shoot into the backfield with wyverns and russes picking up the slack as units move closer.
By setting your fires to support each other, this helps give you freedom of maneuver and set the stage for forcing your opponent to fight where you want him too. Using mobile forces, you can counter attack after your fires have weakened him, or counter any flanking moves they make. Something like below
Hope this helps and gives some good ideas.