[serg3d]

I'm not very expirienced player, but I've learned some tricks:

Most importanat:
In firefight if one side is moving, and other is not, unmoving side have enormous advantage. So try to get moving enemy stamble onto your unmoving units.

Others:

Concentrate your forces

Then positioning your unmoving units try to use edge of the forest/smoke to hide your units, reverse slope defence, and analogus to reverse slope defence - enemy moving from the edge of the forest/smoke into your unmoving units.

Use ATGM versus armor, infantry and artillery versus ATGM, armor and artillery vs infantry, aircrafts and gunships versus armor.
Always kill ATGM first. If you are using air/gunships try to suppress enemy SAM with SEAD and artillery before.

If enemy don't have termal vision (vision 40) use smoke to hide your units while advancing.

Evoid quicksands if fighting in the desert.

Use ammo trucks behing your units if defending. Don't forget resupply your gunships.

Locate enemy artillery by it's smoke plumes and use your artillery on it.

Pay uttension to "Vision" stat of the units, your and enemy. Units with vision 40 or more can see through the smoke.