This can be done with the flamethrower, or the piglet grenade launcher with incendiary rounds.
With the flamethrower the best place to try is at the end of The Birth of Sky heist, when the gang must run through the sewers. This is when an unending assualt of enemies pour into the sewers and bunch up together, allowing you to blaze through them (not apologizing for the pun because it took me an hour to come up with it) and have ten of them burning simultaneously.
With the Piglet, modify it to shoot incendiary rounds and start the Jewelry store heist on normal. Make sure to buy the "Vantage Point" asset as it makes it easier. Break stealth and climb to the top of the vantage point across the street from the store. Wait for the main police force to arrive (shouldn't take more than a minute or so) and they will start to cluster around the entrance of the store and police cruisers. From there you can rain fire on the area and set them on fire.
NOTE: The above method is how I got the achievement and is in my opinion the easiest.
The last way is to start the Watchdogs heist with the Piglet, and open fire once the doors open. The issue with this one is the large group of police (the one you'll need with shields and street cops huddled together) doesn't always happen so you may be left with only 6 weak cops. Restart and try again.
It didn't take me too long to get this but I hope this helps!