Only for some players, this achievment is glitched/locked and will not open. The objective: "Use the Harpoon Gun to pin 5 enemies to a single vehicle." I have done this over and over again. I've repeatedly had NINE guys pinned to a vehicle, but still nothing.
Strategies include applying a liberal definition to "enemies," because some players have reported success with this achievement by pinning non-enemies such as peacekeepers and pedestrians. Still nothing.
Others who have reported this problem elsewhere have said that the only solution is to obtain the achievement through the Crackdown 2 Demo. It is said that, once earned there, it will pop once you load the game proper. But this strategy is a pain in the butt! The Harpoon Gun is a level-4 weapon, requiring the player to level up their character quickly within the demo's half-hour window. This translates to a frantic 25 minutes of character leveling, and 5 minutes of actually trying for the achievement before the game shuts you out.
Who else can't get their Pin Cushion cheevo unlocked? Very frustrating. A fix is in order!