This was not popping for me when using the execute command on a downed enemy.
You need to use the chainsaw command on a standing enemy. You'll need a support character with a chainsaw lancer such as Gabe, be careful of your action points if there are other enemies threatening you as you may leave your soldier exposed.
Also, beware of enemies that can attack you when you get close, for example wretches and grenadiers. They attack when you get close and can interupt you meaning you don't get the kill.