Blasphemous

For this achievement, you just need to assent to Viridiana's help on three separate boss fights. Viridiana will always appear in the room directly before a boss fight. She can help you with Ten Piedad, Tres Anguistas, Our Lady of the Charred Visage, Exposito, and Melquiades. Make sure that you assent to her help in at least one of the three bosses before the bridge of the three cavalries door opens - as you will lose your chance to get her help on three separate ones by the end of the game. She does not appear for Quirce, Crisanta, or Escribar.

Also - there is no benefit of going through the whole game without her assistance. If you do this, she will give you a prayer at the screen before the final boss fight. She gives this prayer to you anyways before she dies at the end of the third boss fight. So, it's better to have her die because you'll have this prayer for the entire game.