Story related unmissable achievement. Marl is the boss encounter at the end of "Assassins Den" located in the top right of the "Forbidden Jails Map" next to that companion cube (power rune) looking thing on the map. One of the earlier encounters.
If you have any trouble beating the encounter, regardless of difficulty turn on all "Accessibility Options"
Having "Accessibility Options" does not negate this achievement.
Happy Gunting