That trophy seemed to take quite awhile for myself as well. What I suggest to do when you throw the rock, make sure the enemy or whoever walks towards the rock and scopes to the scene to make it work. If you're still feeling a bit cautious, then let them walk back to their original spot to be 100% sure it worked.
Popped this evening Scotty. Should have most of the SP done this evening then just the 4 co-op missions to go. When you're ready