The key to me against tanks is , they can fire to you at much more distance than you, and if there are 4 tanks, you can engage one but there are other three against you.
So I allways try is find the correct place to engage one tank at the time and use terrain, buildings what ever to keep you in covert from the other tanks, many times is difficult to achive and take a lot of time but it can be done.
Sometimes a good solution is engage while moving, climb and descend, and only go direct to the tank when you are ready to fire.