Hey Jakub
Let's start with grenades. You need 3 grenades that explode either behind the turret(on the tank) or better, just behind the tank. You don't need 2 or 4, exactly 3. I don't know if impact grenades work, never tried.
We always try to hit the tank from the back when it comes to bazooka shots. One good shot is necessary. On Libya 2, the tank in the valley is many times shot with a bazooka from the side. It's a bit hard, you have to hit somewhere in the tracks or between turret and main body... if it makes sense. It's like you need the explosion to hit some soft spots, preferable in the same spot - just like grenades. From the front seems harder to kill a tank, but doable if the explosion lands somewhere under the front part.
It works the same with shooting by a flak gun.
The best way to kill a tank... works every time
There is another possibility to kill a tank and that's by weapons. You need 6-8 shotgun shots behind the tanks turret to catch fire. Norbert knows of a way to kill the tank with the MAS rifle
The most reliable way to kill a tank? Sicky(Explosives), pak cannon or anti-tank mine