There are several ways to get this achievement. The first is when fighting a Machete Champion, let it lunge towards you. Once it hits you there will be a fireball that sets you on fire. When on fire, lunge towards other enemies to set them on fire in return. Lunging towards the Machete Champion does not count.
Alternatively in the first section of the first level (Unstoppable, Origins) you have to crash a jeep through a door. By doing this you will light two enemies on fire. It will take you about ten minutes every time to get to this section and you will probably light at least 6-10 enemies on fire in one playthrough, even if you are not really focusing on it. If you decide to replay this section a few times, be sure to continue to the next checkpoint, otherwise the game won't save the kills.
However, the best method is when in level three, to load section three. You will start in a courtyard with one or two Shifters (dependent on difficulty you play on) and a spawning machete enemy. First, take out the two Shifters, as they prevent you from lunging, do not kill any of the machete enemies. After taking out the shifters jump on the pad that lets two columns rise up in the courtyard. Jump up to the highest column and lunge towards the spawning machete guy. Do not kill him! Move on to the block with the fire that is off to the left. Kick it off, and jump on top of it. This way you will be set on fire. Next thing to do is light other enemies on fire by lunging towards them or just making contact. As you have a spawning machete enemy getting at least ten would be no problem. If you're smart enough and don't light the spawning machete enemy on fire (he will sometimes jump down after you have lunged towards him), you should be able to get the achievement in one go. Otherwise, replay the section one more time.