For this achievement you will need to complete the Seraphic Gate's final boss the Ethereal Queen in the Infinity Mode difficulty.
To unlock Infinity Mode you will need to complete the main story of the game on the Normal difficulty, this will unlock the Hard difficulty. Then you will need to play through and complete the story on the Hard difficulty, this will unlock Infinity Mode.
Note: You do NOT have to play through the Seraphic Gate on Normal or Hard to get this achievement.
Some tips for fighting the Ethereal Queen can be found here (LINK) and a guide here (LINK).