I'll try to start with the basics, that I'm sure most people know, but maybe some new comers have yet to learn, and to save them from a nasty experience I'll write down what I know, please add if I missed something.
SCOUTING
In dark games it's quite important to know where your opponent units are, to be able to adjust your plans and movements according to your opponent.
I nearly never use it tbh, but if you want to be a good player you should.
The following factors can help you scout effectively:
* opponents starting position and units (obviously, where they possibly can be, i.e. the movement rate of the different units and what he might plan to do with them)
* knowing your opponents "personality" (i.e. the way he tends to play, aggressively, defensively, crazy, straight up camper...


* last seen spots, from the gray ghosts you distract some info in similar fashion as mentioned above, with the added help of a narrower time frame (i.e. how much seconds did that turn go on after you spot him), the direction of the unit and seeing an unit in a specific spot can help you refine your estimates about his plans and personality.
Apart from the obvious (i.e. it's handy to know what your opponent is going to do), the things mentioned above help with scouting because scouting is on one hand a trade-off between lose of time and opportunity and helpful information and on the other hand, you want your information to be as actual as possible, for this reason scouting on target is important to minimize the one and maximize the other.
Then to the practical basis for scouting, I usually scout in the following ways;
* To look around a corner: Cancel aim on your unit (this makes him run faster), do CoS (i.e. continue on sight) on your unit (this makes sure your unit won't stop to engage in a fire fight, where he'll very likely be in a disadvantage), order your unit to run just a little further then the end of a wall and apply a quick-look on the way point or line in the desired direction (quick-look: hold Ctrl > click on empty space), then run back to to safety behind the wall you were standing behind.
This instruction is specifically aimed to look around a corner, but you can use mentioned ingredients in all kind of situations of course, for example stand/duck behind a window.
* Another way to scout is to stay on certain "overview spots" for a while. You can make a unit wait at a spot by dragging the white dot next to the way-point to the right, you'll see a time balk appear, this indicates how long that unit will stay there.
If you do this, make sure you do not engage in disadvantageous firefights, for example machine gunner against sniper over a long range, or machine gunner against another machine gunner behind cover.
The way to do this is by ignoring mentioned units or zones and not stay too long on that spot for your opponent to be able to kill you.
* Run around, surveillance, using quick-looks to check multiple spots, multiple times.
DISTRACTION
To win time, and gain the upper hand in an engagement players sometimes use a form of distraction. It's a bit cheesy imo, although I always use it when I can, but when everyone knows about it, I think it can be an integral part of the game.
Basically you let your opponent's unit focus and aim on a "dummy" running across his "Field of Sight" (FoV), while at the same time bringing your other unit in position to engage the opponent's unit currently aiming at your dummy, this will make sure you have the advantage of "first sight" and all other things being equal will win you the firefight.
How to use a dummy: Cancel aim on a unit, do CoS and make him run into the FoV of the opponents unit. Usually you want to use your fastest unit as to direct the aim of the enemy unit as far away from the spot your second unit will appear on the stage. Make sure to run back to safety in time, you don't have to sacrifice this unit, usually the "bought" time is enough for your second unit to win.
How to exploit the gained time advantage: Order a second unit to target the distracted opponent's unit, he'll gain first sight when applied correctly, make sure he is not in a disadvantage in other ways (i.e. cover or long- short-range against sniper/shotgunner)
* use distraction when you want to take out an enemy unit which is otherwise harder or impossible to take out.
* when you want to get a slower unit across a certain space.
* to delay the movement of your opponents forces (i.e. to a critical point you want to cover first), by letting the enemy aim he loses time while standing still and aiming, thus not moving to the desired directed spot.
CONTROLLING THE GAME
When I start a Dark Extermination match I always look first for spaces I can control and deny to my opponent, and how much I can contain an opponent by guarding the exit routes of his units.
If you can completely contain your opponent, this is good, because you can decide whether to attack or not, how and when.
Sometimes containing just 1 unit can also be helpful, but ofcourse you'll have to be careful for his other units to come to the rescue.
If a contain is not possible, denying your opponent space is the second best thing, this way you can force your opponent in certain positions or to move in a certain way, or to stay still, this will help you predict his movements and adapt yourself to them.
In general I would say, try to expand your advantageously covered space, and only abandon this goal for a near certain kill.