Achievement glitched for me in main game but DLC 'Seeds of Evil' fixed it. Not sure if it's true fix or just workaround devs wasn't aware of but anyway I went through DLC story until I got one new artifact and returned to Arc, then explored again locations from main game, couple of them had respawned enemies and artifacts: 'Lair of The Horned Devil' and 'The Unlightened'. I get key from the latter, open vault in 'City of Lights' and once I killed Lux achievement poped. I already did it in base game some time ago but id didn't trigger the achievement then, maybe because it wasn't my last achievement (getting all artifacts was) or this specific achievement for killing Lux doesn't register.
I started fresh game with ticking on option 'Start Seeds of Evil' with main game characters' or sth. like that which gives you Lv. 61 team, I just expleroed old locations again and leveled up a bit on respawned enemies.