Requirements
Never skip the basics
I expect you to have done around Sierokarte’s Academy Dark Stage 5 at this point. Your grid should look something like this.
Get Bahamut Weapon from What Makes the Sky Blue side story and the Dark Opus Weapon (Katana of Renunciation) from Created by the Stars, Loved by the Skies side story.
The free Dark Opus Weapon is just for one element, so if you picked another element, it is perfectly fine. You can get it later once you reach Rank 120+ and farm Dark Rapture. Can jump towards that section for a bit if you’ve reached Rank 120 if you want to prioritize Dark Opus.