Crackdown

As basically everybody else has said, the easiest way to accomplish this is to use the Cloaking Device to sneak past the majority of the gang members guarding the Generals and Kingpins.

Equip yourself with that, the Harlington HMG-90, and the Firefly. Then, get as close as you can to the Generals with the Cloak. Be fast! If you find yourself losing cloak too quickly, find a place to hide for a moment. The shield regenerates quickly if you stop the Cloak before it runs completely out.

Once you get to them, unload your Firefly on them quickly. Almost every General/Kingpin takes 6 shots of the Firefly to kill, so you can take the time to reload or switch to the Harlington once you've used all 5 Firefly shots.

Almost every single one of the Generals is really easy this way. The only one that's going to present any problem is Wang, and he takes more patience than skill, really. Use the method of climbing the tower in the video below (same video Canad1an Fr3ak posted. Basically, you use the building with the red on the RIGHT of Wang Tower, then jump over to Wang, aiming above the windows closest to the front of the building. You'll land on an "imaginary" platform, allowing you to get into Wang at the floor just below the yellow cylinders), and that'll allow you to avoid an enormous amount of the enemy encounters. This is the GUARANTEED HIGHEST point you can get into Wang (trust me - I found a way to get way higher outside - almost to the top! - but you can't get inside from there, nor can you make it all the way up).

When you get into Wang Tower, hide often! And wherever you hide, it's also best to click the Left analog to crouch because enemies that may come around a corner and see you WON'T as easily if you're crouching. You can hide in plain sight sometimes this way!

As I climbed the tower, I would get past a single major obstacle, then hide and crouch to recharge. Then I'd get past the next obstacle, and hide. I did that all the way up - one by one - until I got to the stairs leading to the roof. One last recharge, then cloak, jump onto the gazebo in the middle then right up to Wang. BOOM! Dead.

My very first completion for Wang at Psychotic difficulty was using this method, and I did it in 5.29.868.

True GS addicts got this before the cloaking device. It is actually easier to get not Co-Op as the Co-Op lag causes enemies to spawn strangely. However, if you aren't very good Co-Op can be useful and unlocks another achievement after completing all bosses.

The best method to get Wang to is go outside and climb up the building to the left(when viewing the front of the building) that has loads of windows. You can them jump onto Wangs Building and you have elimated 50% of enemy encounters.

Hothead is your best friend, some bosses can be killed in under 10seconds.

I was able to get this Achievement rather easily when using the Invisibility power up instead of grenades. Simply turn invisible, run as fast as you can to your target, and pump them full of rockets or lead.

Using this method, the only Boss that proved any sort of challenge was the Main Shai-Gen guy, as he is kind of hard to reach physically. The following method works well for him though: Cloak, run as far as you can, when your cloak is about to run out, HIDE! Let it regenerate, repeat until you get to him, then blast him as normal.