Everything is not math but often what you describe as not being math related is just because you are not doing enough math.
Area denial: risk assessment that probability of losing in that area are greater than winning.
deathrain better than fireknife to kill marines: Take a sample of the number of time each gun fire into a game and use it to calculate efficiency. The number of sample needed can be calculate with statistics for the number of variable that you include.
"My MathHammer, The Weird Stuff that Shouldn't, But Happens to Me!" : Don't think average and probability are the same. by example 4 stealth suit fire at marines. In average you will have 1,33 dead marines. but 1 time on 4096 the 12 shot will hit, then 4096 time on 531441 your 12 shots will wound and the marines players will then 4096 times on 531441 succeed to keep his men alive. There is 1 times in 100 millions that it happen that your 12 shot wound and you did not kill even one marine. But when you roll the dice it migth happen and this not against the math
We take a lot of decision on experience but often those decision can be proven by probability and statistics if we take the time to do it. But who take the statistics about the number of times each model shot in a game, the number of times you had to roll armor save, which model survive at the end of the game and which die, the amount of area of ruins and forest on the board and other data that we need for a real calculation of the math that we need? We are not doing it because it would be a big task but with it we could then calculate a lot of the decision that for the moment we spent with our experience.