There are a few pingers in the game. I counted 6, but seems you only need to kill a specific set of 3. Pingers are the HUGE robots on 3 legs. On the 6th mission, the one where you need to take out the 3 AA guns but touching the 3 barrels, there is a set of 3 pingers that you need to kill. When you see them, check around, there should be a secondary mission, called something like "rescue motor crew." You don't have to, but if you, they can help you kill the pingers. I did it on super soldier with nothing but all the rockets sitting around, so it's certainly not all that hard if you do it on easy. There is also a buggy or two sitting around you can use with its rocket pods.