I'd say, get a good cover. I like boxes, you can keep pressing control so they won't shoot you (unless they come from the sides) and you can just release it to shoot at an incomming enemy. Now at 0.11 sandbags should also prove to be great cover position(didn't really use them in 0.1 because most of my shots wouldn't go further than the sandbag in front of me XD)
When you don't have any of these and you have to take cover in a building, a big rock or a huge blue crate just try to do this, go to the corner, aim at an enemy before you leave it, go out a bit, shoot it, get back to the cover, reload and do it again. If they come to your cover, don't run unless you see that they'll come from both sides because if they don't you can just spray your clip at their side while you move around your cover to get back your advantage.
Also you should really try to aim your shots against the actual enemy for a more precise firing

When you're fighting a large group grenades are indeed helpful but try to predict where they'll move to and throw a grenade there (use more grenades to various possible routes if you feel like it but you'll have to aim them fast XD)
This is kind of how i play, though i do mostly solo suicide missions. You should follow a group if you're feeling unsafe but still, these things can still aply

Now go kill some enemy soldiers
