After completing the main story, a wealth of post-game content opens up, offering the ultimate challenge. This includes new, extremely difficult challenges, higher-level versions of existing missions, and the ability to replay any mission with any unlocked character. The 'Apocalyptic' difficulty setting also becomes available, pushing your skills to their absolute limit.
Focus on maxing out your favorite characters, optimizing their weapon builds with perfect seals, and completing all remaining requests and challenges. The post-game is where you truly test your mastery of the combat system and character synergies. Many of the rarest materials and most powerful weapons are found in these high-level missions.