There's one of the Trials of the Rykiat missions that is a good place to grind out the stone throws, can't remember the name of it but it's the one where you start with just a silenced pistol.
I need to throw more stones. I'm on the last couple of missions on the 2nd island, nd it's looking likely that the only achievements I won't have are the stone throwing one and the 6 co-op missions. Unfortunately I've killed every hostile outside of the story missions so there's no one left to distract. I'll have to try getting it by clearing up the last few Wanted missions I suppose.
All the eurogamers who actually play with each other on xbl rather than just post pseudointellectual pc handwringing bollocks on the forums, love the shit out of biggy.