A friend and i managed to finish this chapter last night and to us it seemed like the disengagement from the tank happens when the camera angle changes.
Without touching the mouse we managed to turn the turret in the general direction of the left side of the valley and then not touching anything but the fire button. This way we managed to take down 5 out of the 6 giants before disengaging from the tank and having to run down and kill the last giant.
Hope this helps someone out…