PAYDAY 3

A good spot to go for this would be on the map "Dirty Ice". Start it up on Hard difficulty and go loud once you enter the jewelry store. In the employee break room where the whiteboard with the WIFI code is, stand up on the table facing the doorway that leads down the hall and place your sentry turret down. I'd highly recommend leveling up engineer enough to unlock the "Spin Cycle" ability that allows the turret to rotate 360 degrees, as it'll allow it to also take shots at the front door of the store in addition to the AP turret Ability, which will give your turret increased armor penetration. You'll notice as the assault gets into full swing that the SWAT will usually funnel down the hallway and lead to many kills with the turret.

If you have the Dual Sentries ability you can have one looking down the hall and another looking out the other doorway toward the front entrance, which will speed things up a bit.

Keep an eye on your turrets and complete the heist as you normally would up until the escape point, after which if you're still not sure you're at the 50 kills needed, just sit in the break room with your AI Teammates/Friends for a bit to mop up whats left. Afterwards, head to the escape.

The achievement should pop on the Heist Success Screen.

Hope this helps!