That is the hardest one...no one seems to use the sentry gun anymore. I think got mine in demolition. In one of my levels I had to play with friends and destroy one of theirs. My friends are jerks and kept killing me...finally got one.
why can't it just be a turret of any kind. like the sentry and the rocket one that takes down air support.
Because the Sentry Gun actually shoots at approaching enemies. And the SAM turret just shoots enemy aircraft. The point is you have to sneak up behind the Sentry Gun while using the Ghost perk. There's practically no skill in killing a SAM turret.
I really think a lot of it has to do with being willing to sacrifice a bit.
While they arent used a lot, considering you only have to destroy one, compared to the quantity on the other two challenges, you are bound to run into one.
But I think a lot of times its easy to forget in the beginning, or think " well we're winning, and I have plenty of other challenges to do ..... sooo I'll just wait" Then when you have everything completed but the sentry gun, you may not see one for
Be prepared ........ they are easy to get, so if someone is equipped, you are sure to see several of them over a few matches.