For this achievement, you will have to kill an enemy affected by the power of the Timefist weapon, which will become available for research after you kill enough Twitchers, around year 150 or so. It's a Caber weapon that grants a special RT power which makes an enemy disappear and come back later. Sometimes just a few turns later, sometimes in a completely different battle! It's completely random. Simply use it on an enemy, wait for them to return, and kill them properly this time. Doesn't matter who gets the kill.