Right, so first off, basic combat.
Combat in this game is on only 2 dimensions, and thus complicated maneuvering is nonexistent, and thus firepower is more important here than in other games, or, real life.
Not to diminish its' importance.

First off, recon.
If you have fog of war activated, you'll want to know what you're hitting. Grabbing a light soldier of any type and handing him a sniper rifle is valid, and probably the only real way to conduct recon other than scanning.
I'd recommend avoiding an engagement, and save your troop for later use, probably in a defensive position.
However, with your advanced range, you can harass hostile troops a bit, wounding them, and kill a few off.

Combat comes next.
In any combat scenario in this game, you should give it your full attention since the AI is lax when it comes to range unless they are the Ronin Sniper, who has advanced engagement range, making them VERY deadly.
Now, you should always control a soldier when in combat, and constantly fire, especially since you can engage farther. You should make good use of cover, if available. You can also make your own in the form of trenches or dead bodies.
There is another guide somewhere on Steam depicting trenches, although it's specifically for WWI mods, the basic use and construction of trenches is the same.

Squads allow you to engage farther as well, with larger amounts of troops, as long as you're controlling one of them.
However, an explosive can ruin this, so be wary of explosive weapons.

Making good use of available support will greatly improve your troop strength.

Defense is much easier than offense.
However, in this game, you'll always need to resort to attacking, unless you're playing certain Skirmish modes.

First off, you'll want plenty of cover for your troops. While, in this game, everyone is fairly sturdy unless you're using certain 'mods' in the mods section in-game, you'll still be happy to have your guys behind cover when that sniper misses his first shot, or you're saved from a rocket.

Secondly, you'll find that a sniper can help a bit, and that you'll be able to pick off some troops easier this way. Also, it'll allow you a little more advance warning and engagement range.

Some sort of Air Defense in the form of heavy weapons on the surface of your base will help, along with some towers/outposts a little ways out to set up a perimeter.
Also, having a good surface base built up is good, too. While a bunker is good, you'll have restricted where the enemy can land, and give you an area where you can bring in you're offensive fireteam/squad/platoon (however many troops you think you need) to attack the enemy brain.

The Air Support and Phoenix mods give exceptional anti air options, along with Techion with a man portable Flak Cannon.

The deployable SAM isn't powerful, but deploying it in the center of your base gives you (decent) long range anti air.