The first time you can earn this achievement is the ‘Dark Star’ level.
You’ll automatically acquire the flamethrower shortly into the level. Once you do you’ll be attacked by mutants while on the train carriage. Soon as they start to come down the tunnel light them up! You’ll most likely only kill 10-15 before the stop coming. Once they stop coming, press start and restart last checkpoint and repeat the kills again until your trophy pops!