The achievement description is inaccurate.
You actually need to kill 50 marked enemies, while you are marked. Once you are notified that you are marked, the opposing player that you need to kill will have a red marker over their head. Killing other enemies will not count towards the achievement.
You need to get the killing blow, as assists don’t count.
I found quick characters with lightsabers worked best for me such as Rey/ Maul. It depends on your playstyle though.