On the Insane and Brutal difficulty levels, the Parasite can survive several seconds of burn damage after being ignited by a Flamer. During this time, the player has an opportunity to punch it without the risk of becoming infested; however, the Parasite can sometimes survive the full duration of the fire and regain the ability to infest a marine.
A safe way to earn the achievement regardless of difficulty level is to approach the parasite while electric armor is active. You can run up to the parasite and punch it to death without worrying about infestation for the time being. Any parasite that leaps to infest you will be destroyed.