UltraGoodness (EP)

For this achievement you need to get the Flammable Bullets upgrade from a chest and kill 10 enemies with it or alternatively you can use the flaming squares that spit fire to kill 10 enemies and that will count as well.

If you're struggling to find the flammable bullets or the flaming square you can load level one and keep restarting over and over again until you find a chest with the upgrade or the flaming square.

I show this achievement in my walkthrough starting at 4:20 minutes in the video.

I have an additional way of unlocking it if it helps.
With the 3rd character you have molotov cocktail on your RB button which can easily help you do this achivement.