The Warlock dwells at the very bottom of the abyss, after fighting your way through whichever path of rooms you decided to take, your character will engage in dialogue with the warlock and the battle will then start immediately afterwards.
It is a good idea to have as much health as possible, as well as health potions.
Obviously any extra helpful equipment you can obtain would be welcome in the fight.
The Warlock is very fast and has a few different attacks but is overall predictable and should be quite easy to find your own strategy on how to kill him.
You will not keep weapons or equipment after this fight and the achievement will unlock after dealing the final blow to the warlock.