Note: This guide includes pre-infamy builds. If you can move up skill trees faster, do so at your own leisure.

Not that most of the skills matter, but there are a select few that should be taken into large consideration.

Bullet Storm (Aced) / Swan Song (Aced)
The ability to fire infinite amounts of bullets will greatly help you not care about where most of them land.

Ammo Specialist
The extra bag will give Bullet Storm users an extra 15 seconds of infinite fire, in case they didn't make the first time.

Gunslinger
Just to save waiting time if the first run was not successful. Aced version will also give more damage in case the 128 was not enough to kill most things in the police force.

Example Build[pd2skills.com]

Finally, bring a Peacemaker .45. Why you would try this without one is beyond me.

Try to find any location with a condensed area of cop spawns. Jewelry Store (Ukrainian Job), Rats, Mallcrasher, Nightclub, or The Diamond (DLC). The difficulty is completely up to you. Playing on Normal will help alleviate not dying, however OVERKILL will give you more officers to shoot. Personally, I tried this with Rats on OVERKILL, so if you want a recommendation, there it is.

When the heist starts, don't go trying to push for the achievement straight from the get go. Give the assault time to build up and for the police to start spawning in. Once they have gathered en mass, place down an ammo bag (Bullet Storm) / get downed (Swan Song) to activate the infinite ammo. Though six seconds sounds like a short time, don't spam fire your revolver. Regardless of mods and skills, it you'll hit less than you'd think, the camera shakes everywhere, and the gun kills people in practically one shot anyway. Just take a moment between each shot to readjust your aim. It's not like it takes very long anyway. If all goes fine and dandy, you should have the achievement pop, a new mask, material, and pattern. Not to mention six dead cops in who knows how many bullets.

Thanks for reading the guide, hope it helps, and good luck out there for all you achievement hunting heisters!