Mindjack

I found level 10-1 to be far by the easiest. At the very beginning of the scene, hang all the way right around the corner, and kill off the 2 gun wheels then mind slave them. Hack into one of them and you can kill quite a few enemies each time through the glass walls.

The 1st level the Gun Wheel is found on is level 2-2 the "Colosseum" scene. There are a total of 5 of them.

If you want to get the achievement legitimately you just need to shoot one, mind slave it and then hack into it to control it then fire it's guns at 20 enemies. Doing this is very difficult because lining up a shot is awkward and once you got the shot lined up and start firing whatever you are shooting at will turn towards you and return fire if they weren't already shooting at you before you started shooting them. It takes fewer shots to put you down than it does just about anything else. So it might take several tries redoing the level to get it that way.

However if you're not above boosting with a partner then it's much easier to host a game and have a friend join as a Red team player. You will have to shoot the Gun Wheel and hack into it then you'll want to move out of the general combat area to a corner away from the NPC's shooting at each other so you aren't hit by them. Now have your friend who is on the Red Team hack an NPC and meet you in that corner. Have the friend stand there and let you kill them then have them go and hack into another NPC and repeat until you have 20 kills. It might take a few tries redoing the level to get it, but shouldn't take more than 2 or 3 if you work fast. Also as the host try not to shoot more enemies than necessary prior to hacking the Gun Wheel because you'll want to have as many available as possible to use to shoot with the Gun Wheel once you hack it.

I went with 2-2 Colosseum, in "Scene Selection". After you load up, kill the first flying turret, then mindhack the civilian at the bottom of the stairs. Use her to melee the 2 guards next to her and kill them, dont hack. (helps towards I KNOW KUNG FU, MOWER MAN, and HONORARY CITIZEN...the AI likes to kill the wheels before i could hack them). through the glass doors, there will be 2 gun wheels that come out at first. Kill 1, hack the other, and get it early so they are as far away as possible from the center of the area. Then drive it as far as you can to the edge of the room. Beware the 3rd that comes out about 30 secs after the first 2. Now that you are far from the rest of the action just keep shooting (zoom in with left trigger) at the guards. you can hit them and they cant (or wont even try to) hit you. This is the key to making the wheel last as long as you can- staying away. I got about 4-5 guys each time this way. The second half of the level there is 2 more wheels that come out, over by the escalators, but they are so close to the action its hard to get a kill with these. Just reload and do this few times. (running up the escalator completes the level, you might need this to save the stats. It felt like i had more than 20 kills, but a couple times i just quit out before finishing the level.)