after snipping we must destroy all the tanks.. but is it only that ? because i destroy at least 4-5 tank and another arrive.. and a nother.. and another.. and they kill me.. calling the airforce on the top of the house is it the only way to accomplish the mission ?

You have to have a view over the bridge and the tanks and watch your back.

Once you snipe down the guys running on foot to the detonator, switch to binoculars. Order fire missions on the tanks. After about 4 or 5, some guys are gonna back door you in the building, shoot them. Go back to the tank area, order a few more fire missions, then more guys will back door you. Once you destroy the tanks and if you survive, you should be all set.