This achievement is unmissable and unlocked with story progression. The final mission is unlocked after maxing out satans wrath.
The final boss's only attacks are fireballs and an unavoidable telekinesis attack that does little damage however remain wary of enemies that spawn on the balconies. Several times through the fight satan will dissapear and spawn clouds of smoke, one of which hide him. Failing to find it causes a fireball to spawn for each of the remaining smoke clouds. Using any fully upgraded weapon should finish the fight easy, feel free to lower difficulty for this fight if necessary as there are no difficulty specific achievements for this game.