**You do not need to start a new playthrough to earn this**
Whilst other guides are correct, this has to be done in one playthrough without loading saves, this method works. This is because the game creates a brand new save at this point, without reloading your old ones.
From the main menu, select 'Extra Verse'. Then select 'Normal' difficulty. When the dialogue choice comes, select to 'fight'.
During the fight with Ardyn, just run to the oppsite end of the arena to him. Either go to the pier or the courtyard, and he will just wander around without attacking you.
You will see every 30 seconds or so, waves of enemies will spawn.
Select the frost spellblades, as this clarity does an AoE attack. Due to being empowered, you one shot all enemies.
to charge your 'Total Clarity' meter. Run to the adds, and press
to unleash the fury. As your counter is starting from 0, this will take about 10 minutes. The adds spawn indefinately, you just have to wait for the airships to drop them off.
Hope this helps, any additions please comment and let me know what to change!