Half-Life 2: Episode Two

I believe you need to use the gravity gun to take the grenade whilst the combine/zombine is arming it, then pelt it back at them

if you've been killing the same soldier who threw the grenade, then the game isn't recognizing it, and you're out of luck, unfortunately.
If you haven't been doing that, then that's what you need to do.

I don't think you need to suck it out while it's being armed. Try to catch it with the grav gun, or throwing it by hand(pick up, run and release to throw using your momentum). I also don't think zombine count for that achievement.

I've tried both ways, using the gravity gun to throw it back at the guy who threw it at me, and another soldier who didn't throw it. NEITHER WAY WORKS. This is STUPID.

Pick up the grenade off the ground. No Zombines here. Throw it back at the same Combine that threw it. That should net you the acheivement