This is a lot easier and simpler than a lot of people think; and if you know about this already, share you input on how to make it easier.

After hours of staying in that little bathroom in Framing Frame Day 1, I realized that you only need to kill people without alerting the alarm, not the person. That being the criteria, we can easily do this with ECM rushing.

Required Items:
Venomorph Mask

Important skills:
ECM Specialist Aced (Ghost 4)

Recommended skills:
Sprinter Aced (Ghost, Tier 1)
ECM Overdrive (Ghost, Tier 5)
Pumping Iron (Enforcer, Tier 2)
Endurance Aced (Mastermind, Tier 1)

Firestarter
Day 1
Start off in Firestarter (offline and on a low difficulty), and speed through Day 1 by taking the minimum amount of weapons and burning/securing them, depending on which is closer.

Day 2
This is the part where you'll work on your achievement. Sprint towards the building while placing your first ECM and rush in to melee everyone. There are 5 guards in total in the FBI office, and a good way to find them is to simply fire off a weapon and looking for the exclamation marks. Place your second ECM roughly 20 seconds after the first one you deployed.

Once the 5 guards are dead, simply restart the day (which is why you should do it on offline mode). This should take about 15mins for 50 kills.

Important to note:
- You could also do this on Framing Frame Day 1, but there's a risk of running into the lasers which might make it take longer.