I started playthrough 4 days ago, but since I had a lot of exams I couldn't finish it then and I decided to leave it till the last day of Endless Day event (which is today btw). I have just finished the final floor with Esseb Tarosh, whom I had since very beginning and... And that's it. No achievement, nothing at all. Talk about my disappointment now. :(

Looks like you'll have to rollback your system calendar to 21 of Dec and repeat your run with Essssseb. That's how I got the achievement.