You will get this achievement for beating the last boss battle in the game.
This can be a difficult thing (even on easy difficulty) if you are not prepared for it - as I was. I recommend to fill up the energy for your void sword and your chaos crawls.
If you struggle at the Boss battle you can always choose to restart the Level - this will place you right before the last chapter. Mark Chupacabra's Shop in your mission select screen and follow the marker. This will bring you back to his shop with the "wolf altar travelling" - you also can fill up your energy on the way or on the way back.
Now buy 3 tears of a saint and you should be good to go. Go the way back as you came and start the Mission.
For the fight itself the most important thing is to dodge the attacks of the boss and try to always place a few hits after dodging. Use your chaos energy when you are close to him to deal extra damage or if needed use the void sword to regain health. If you are getting in trouble use one of your tears of a saint.
The fight itself will be split up in direct combat and a few segments where you have to avoid his "lightning bombing".
If you have tips to improve the guide for higher difficulties feel free to comment on the solution.