How do you go to battle?

[Weasel]

"If I do have that one hill what is the best thing to plan for? placing accurate long-range guns there? delivering infantry first to take the hill and then move on to...? deliver some arty shells first?, flanking that hill with armour first and then advance infantry and then the guns?, etc."

Well in this case, controlling a single hill would probably be deadly to you. If I were up against you, and you had control of the single hill in the game I would pump every round I could into it, making it unusable for you. A simple and true axiom is "tanks take ground, but only infantry can hold it". Anytime you can isolate an objective from reinforcements you are doing well; so yes you could surround the hill with tanks/infantry to cut off reinforcements while you assault forces move in under artillery.

"Guide says regarding the battle plan: "It is general enough to be flexible but specific enough to be executed": When would I know my plan is not flexible or specific enough?"

An example of not being flexible would be deploying all of your units in one area, lets say a map edge, with no ability to move to any other part of the field quickly. An example of being flexible in the above would be deploying all your units on a map edge, but having a lateral road that allows for high speed reposition with mobile forces to counter any sudden threat. Thus, you can achieve the objective, but also be able to achieve the objective with those forces while moving parts thereof to react to any unsuspected situation.

All in all there is no "do this and you will win". A simple and good rule of thumb is to buy and deploy your forces thus: 40% of points for main assault/front line, 40% for second line assault/reaction force, and 20% as reserves.

There are many AARs on the blitz too, I know I have posted 4 with various opponents.