Sniper Elite 3 Walkthrough: Halfaya Pass Part 3
With all but the last cannon left head over to the left side and take the hidden cave entrance around to get closer and take out the guards quickly and quietly.
Climb the watch tower and kill the other guard (sometimes 2 gaurds so be careful) then snipe the red gas canister on the far left of the cave across the raveen.
With the cannons destroyed and all objectives complete, head back to the tower with the radio broadcasts. BUT FIRST swap out your sub machine gun with the panzershrek (rocket launcher) you get off one of the dead soldiers in the tower. You will need it.
You will trigger a cutscene and a tank will roll in. Fire on the tank to disable its treads and then lob 2 grenades at it to take it out quickly.
Congrats! You beat Halfaya Pass!