Just a little note, if you have ghost equipped and you destroy a sentry gun in any way, shape, or form, it counts towards the pro perk. I just destroyed a sentry gun with my Chopper Gunner and completed the challenge.
I'm assuming it also works for mortar strikes, rolling thunder, gunships, and napalm strikes as well.
LIVE is still the internet. And people are ass hats on the internet because you can't punch them in the face for it
It's not just a sentry gun, it's a turret kill so you can destroy a SAM turret as well. What I do is I buy the sam turret and throw it into the enemy spawn. They usually put it back there and I just go back for it and knife it.