He will win who knows how to handle both superior and inferior forces.
-Sun Tzu, the Art of War

They key to any plan is to get some form of superiority over your opponent, and then exploit it. No one ever went into a fight thinking, we'll win because they're better.

So, what sorts of superiority can you get and exploit? Assuming that the game designers of the game have correctly assigned point-values to each unit, your opponent and you should be about even, with neither of you having an advantage. So overall, no one is superior. Sometimes, the game designers screw up, and some units may be under or over-priced, but we'll deal with that later, for now, assume that they're all perfect.

Phase Superiority

Well, there are three phases to the game. So, it stands to reason that you can get superiority in some sense relating to each phase. You can have superiority in movement, firing, or assault.

Local Superiority

The board is also a big place. If you subdivide it, you can then examine each section separately. While both armies may be equal in points overall, within any given subsection of the board, one army may have more or less points than their opponent. This is refered to as local superiority.

Troop Superiority

"It is not big armies that win battles, it is the good ones!"
- Marshal Maurice de Saxe

"Everything counts in large amounts"
- Depeche Mode

To further add to the question, you can have superiority in either quality or quantity of troops. A space marine is a very versatile troop, with overall high stats. But, such quality troops come at a cost double, or even triple that of other inferior troops. For the price of one marine, you could buy three spinegaunts. While an individual spinegaunt is unlikely to accomplish much, thirty of them are a daunting foe, and ninety make a fearsome army.

The key to winning a battle is to correctly identify which areas you have superiority in, and to exploit that superiority, while preventing your opponent from exploiting the areas where they are superior to you. In other words, you want to take your strengths against your opponent's weaknesses, while prevening them from doing the same to you.