This achievement is unlocked in the second area Sett Maat.
From the first prayer statue in Sett Maat take your first available right. Then move forward to the second NPC you wool meet
Sherybin he gives you the quest to kill Senen.
I would advise being at least level 4 preferably level 5 before on this boss. Make sure you also get the two gems available one from Pratchnefer as pre payment for finding her daughter, another from a man who's property was stolen.
To find Senen
Take the path between Sherybin and the unemployed craftsman. Kill the spearman and take the right before Pratchnefer. Move down the path and combat will begin.
Make sure you have 3 canteens this guy hits hard and his movements are not as predictable as the Warden.
I employed the double roll method, rolling behind him waiting for his attack then quickly hitting him slowly chipping away his health.
Once he is dead walk past his body and look for a lever. Interact with the lever to open a gate. Until you pull that lever you can't complete the quest. Check in with Sherybin to claim the reward as well as this achievement