It is best to start working on this early in your playthrough, as enemies don't always die from the fire when they're higher level.
I worked on this while completing locations for "Old Habits", I would run a group of enemies around the location and shoot an oil jar when they were all standing next to it. If the enemies get stunned by the oil jar exploding, they're more likely to die from the fire. Do this quite a few times and eventually you'll hit 30!