In Mission 6: Only Human, there are three total Pingers. All of them can be found on the way to the Defense Battery Bravo main objective. Pingers are basically tanks on legs. There are lots of explosives in the weapon caches of this area. Use the rocket launchers and C4 explosives. Your Predator Bow will also work well. The explosive arrows and electric arrows are very effective. If you are having trouble, set the difficulty to Recruit.