After finishing the game, you'll unlock the last specialty called Limit Break. This will increase the level cap of your characters and their battle skill level as well. (Up to LV255 for characters and LV9 to battle skills)
After the end-game credits and ending scenes, load the last save then travel back to the entrance of Kronos’s Sickle to return to the ship. You can now access the Maze of Tribulations from the ships’ transporter. Please refer to this guide's dedicated section for this post-game optional dungeon.
If you want to return to Kronos’s Sickel to defeat Alma again and view other endings, you can travel to Sthal and use the rift inside Fidel’s home to go back to the first floor of Kronos’s Sickle. Of course you have to travel all the way back to the last savepoint.