On the Dark Star mission, you'll pass out and have a vision of the Dark One. When you wake up, your middle weapon will have been swapped out for a flame thrower. This is as good a time as any to knock out your 30 flame thrower kills. Just get between 5 and 10 kills, then reload the checkpoint. When the enemies start slowing down/thinning out reload as you may finish the section before you realize it.
I've created a full Enlightened playthrough video showing almost every Moral Point in the game as well as pointing out items of interest, such as this achievement.
You can see the checkpoint I'm referring to in the Dark Star mission for the flame thrower kills on the railcar at the
1:48:04 mark in the video.