Hey Kurgan !
First of all, you've come to the right place! Second of all, my predecessors on this thread have made very valid points worth keeping in mind.
I like the fact that you seem to reason with principles to guide your actions. Very Ultramarinish ! I've got a set of principles that you will get to enjoy quite a bit and will most definitely help you on the battlefield, as they have helped me tremendously.
Will add a very brief description of the logic behind each principle.
Principle 1 : For every enemy a unit can shoot, these enemies can shoot the unit down. It's always preferable to avoid exposing one unit unless absolutely needed, as it implies that this unit is exposed to return/counter fire.
This is pretty much what you've experienced on your heavy support squads. Always use this principle to keep in mind that the opponent will react to the most priority threat. Cover is always misleading in tabletop games, as we tend to picture it as laying ambush with only the gun barrels pointing out. The game mechanics tell a different story, with people standing up and a piece of wall just happens to be in the way
Principle 2 : For every unit that you deploy, you are revealing more and more of your strategy to the opponent. If a unit's presence on the battlefield does not yield to advantages both immediate and long lasting, you will benefit more from holding it in reserves, then deploying it where and when most appropriate.
This will help conserving your forces and giving you some flexibility throughout the game while not revealing your entire strategy to the opponent. Even Heavy Support units!
Think of the following : You keep your Devastators in reserves and in a Razorback (yes, I went there), and once the game has developped, you deploy them when and where it will hurt the most, using your range advantage and now seeing where the enemy has committed his forces.
After all, what is wasting 2 turns of Devastator firepower if they are wiped Turn 1s usually, right?
Principle 3 : Don't roll dice unless absolutely needed. If you do, ensure that your strategy is minimally reliant on the outcome of said dice.
This is why 2+ rerollable is what is looked at by competitive players. In other words, just make sure to base your strategy so that bad dice rolls are not going to give you a hard time.
Principle 4 : Your units are only tools to lead you to victory... And a good craftsman always knows which tool to use for which job, and preserve them over the longer run.
Don't expose your units when not needed. It's better to avoid risking getting shot at than throwing a unit with a terrible probability of doing damage.
Principle 5 : Always strike to find the good balance between combined arms and volume of bodies. You need enough variety but also enough resilience for each of your 'arms'.
In a nutshell; you will gain from presenting different kind of threats to the enemies and this increases your available arsenal. By different kind of threats, I mean vastly different ways and means based on unit strengths, such as :
- Infantry : Small visual profile, limited weapon carrying capabilty with short range, best operated in confined environment where they can operate without being shot at
- Vehicles : Larger weapon capabilities, very resilient to small arms fire, higher speed. Best used in environment where armour, speed or range gives them immunity to infantry
- Air : Faster moving with high payload, but little survivability against dedicated weapons. Best used in cavalry role while minimizing exposure and maximizing fire concentration
Principle 6 : Directly from Sun Tzu, "First, look to render yourself immune to defeat, then look to exploit your opponents' weaknesses".
No army is immune to defeat, but if you make it the hardest ever possible (concealment, counter units, etc) for the enemy to come and kill your units, you're offering him many opportunities for him to make mistakes.
Example : I recently won against a Tau friend by only deploying 15 Marines on the table Turn 1, in deep cover and line of sight blocking element, and giving him the first turn. He ended up frustrated at my avoidance tactics and forgot my reserves. He dropped his Dakka Broadside unit in a position out of cover, primed for getting shot at by my two Vindicators that came out of reserves on Turn 2.
It's just a few to begin with, and some of them might seem simple, however they are very interesting to keep in mind at all times during a battle! I can guarantee that you will see many benefits remembering these principles, and soon enough you will start to visualize new arrays of tactical opportunities on the fly !
And it's okay if they are simple! Did you know that the word "principle" comes from the latin "principio" which means beginning ?