Crackdown

Although they state that you have to attach 5 “GANG” members to a single vehicle it will work with anyone. I would suggest using a large vehicle to make things a little easier. Now simply drive to a populated area, park up, and the when someone walks past – harpoon them to the vehicle. If you feel the authorities are getting too anxious and start to come after you simply jump in the vehicle and drive off till the lose interest. You don’t have to worry about those already attached to the car – they are not going anywhere.

Requires all DLC.

Much easier if you attempt this with a larger vehicle, the APC is probably the easiest just because of the fact that it is the largest. From there obviously you need people to attach to it. Your best bet is parking it near the curb in a crowded area, crouching, pointing your weapon at it, and firing whenever anyone walks between you and the vehicle.
If you have a co-op partner this can get a little bit easier as he can drive around the crowds slowly as you peg them to it.

Here's how I got this one: Go out with the harpoon and Firefly grenade launcher. Locate a fairly busy street and just start blowing up everything civilian. The point is to get the cops to hunt you down. Once they all arrive, switch to the harpoon and lock onto a cop and shoot them to pin them to the police car... rinse and repeat.

As another commenter mentioned, the Achievement description is wrong; you can pin whomever you to the vehicle.

More of a tip, really:

If you have a partner for Co-op who has this DLC, but you do not, you should have this friend host the game.

You will gain access to all of the DLC content and can get this achievement (and the others, for that matter) without having to purchase the DLC yourself.

Perhaps the easiest solution is this:

"BE CAREFUL IT REQUIRES ALL DLC"

From the tower of the agency you choose a large vehicle (The APC or similar in size), get off the elevator but do not take any of the tunnels. In this area there are about 6 guards, 2 for each tunnel so we have 3 groups. Place the vehicle not too close to the first group and from behind the guard shoot him with the Harpoon gun and attach him on your vehicle. Repeat for the other 2 groups, and in less than two minutes, the achievement will be yours.

Easy exactly as it sounds, good hunting.

This is how I got it, and you can take you're time with this as it requires a lot of patience.. Equip the lobber or Firefly and obviously the harpoon gun. when you exit tunnel 1 from the keep there's a gun battle going on between the agency and Los Muertos gang members. Take everyone out and you'll notice there's a truck parked next to an archway in the wall. get into the truck and park it so that it is side on and right up tight against the wall.. Now go lift a body and carry it, jump onto the back of the truck an DROP not throw the body onto the truck. Because it's parked against the wall, the bodies don't slide off. Just jump into the air over the body and shoot directly down onto the body part that's over the truck. Like I say it takes a lot of patience as it doesn't work with every body, but gang members or Agency bodies count. I got this in about 10 mins. Hope this helps and good luck!