Here is an easy method that can be used very early on in the game to liberate all the Bell towers in Far Cry 4. What you do is pick-up a buzzer - they rez at the very first outpost you liberate. Then all you need to do is fly from tower to tower and park the chopper on the very last level of the tower. Apparently there is enough room and if you get it right, there is no need to kill anyone or climb all the way up the tower.