A Shotgun (Obviously)
Personally, I recommend something with high damage. A Reinfeild 880, Raven, or even a Moscani 12G can fit the bill.

Below this line is all equipment recommended, but not required.

000 Buck Ammo
Self explanitory. More damage, less chance of enemy survival.

Pumping Iron (Basic)
Now how could this help you in not missing? Simple, the game's accuracy only counts for shots fired, allowing melee kills to go unhindered towards your kill count.

A Strong Melee
Everyone has thier prefrences, but I would go with the Kreiger Blade. It charges quickly, and can do at least 100 damage with Pumping Iron, which is the health limit of all enemies on normal. (Save for a Tazers, Cloakers, and Bulldozers) If you don't have any of the DLC (Your achievement hunting days are already well over) Try to use the Tanto Knife, which is similar enough to fair some damage on normal SWAT units. (If you aren't part of the PAYDAY 2 community group, then why are you achievement hunting for this game?)

There is no real specific place to go that would provide any benifical aspects to you, but a quick single day job, such as a Bank Heist or Four Stores. That way, once you get your 50 kills, you can escape easily afterwards. A difficulty of normal is also recommended for low health and damage enemies and lack of any major specials. This keeps your kills quick, easy, and not ruined by that Taser you didn't see.

If you have ever done the Swing Dancing achivement, you'll find this much similar and in the same vain. All you need to do is to achive 50 kills, using only your melee weapon to avoid any chance of missing. This will be a long tedious process, and health kits would be most useful to bring or bought as assets. Don't try to seek every target you see, and let the cops come to you. Be aware of groups, they can be deadly as your meele isn't instant kills for everyone. Make sure to keep your armor up before engaging and re-engaging combat, you may seem invicible sometimes, but it only seems that way. Once you have killed 50 cops, either by counting, guessing, or using other formittable tools like HoxHud, find 2 cops or more and try to line them up. It may be a shotgun, but it doesn't spread as far as you want. Make sure the cops aren't perfectly lined up either, or your pellets won't hit the cop in the back. Just keep the other cop a little bit to the side and pull the trigger.

If the two cops are down, then congratulations! But you still need to head to that escape, so don't go celebrating and shooting just yet. Once it's all over, check your personal stats and brag to all your friends how you killed 50+ cops with an accuracy of 200% or something or other.

If you can hold your own against cops, then this should be as simple as that. No need to shoot, no need for a large amount of double kills. Just melee prowress and one good shot. Hope this all helped you heisters out there! Good luck!