Couldn't see what I was shooting...
Except for shooting down rockets and grenades, I found myself just shooting and dropping grenades randomly hoping they'll hit something. When I tried to get closer to the ground in order to aim I just took too much damage, and enemies took too much shooting to die; aiming my grenades was really difficult and enemies anyway run around too much...
This game has a good basis but usually I'm much better in such games. Maybe it just needs more powerful weapons which are easier to aim.