Im about 3/4 way through a second Post Human Warrior playthrough and I killed the first Pinger in well under 30 seconds, infact it was probably under 15 seconds. I immediately Cloaked, ran up to it, put 4 C4 on its back and took it out with the detonation.
The Pinger, along with the entire game, is not difficult regardless of setting. This is one of the easiest FPS games ive ever played.
lemme guess... you rushed it and btw i like your orginallity of killing it...
How you choose to kill a Pinger only changes how long it takes to do so, not how hard. Cloak makes the game a cake walk.
then try to kill it by only armor mode

see if its easy...