I am about to reach rank 100 and I can read that the Twinkling Stars quest requires a fight with the Omega primal beasts by your lonesome. Just to be clear, alone as in your party of allies in solo battle (not raid battle so no calling for assistance)? Or alone as in the ultimately daunting task of epic Herculean might requiring you, yourself, and I, the main character, to face off against the Omega primal beasts, with only your weapons and summons?
That would be arguably more difficult than the Search for an Heir quest. That quest requires a fight mono to mono. Off-topic, I couldn't bear losing in the Chapter 9 Quest: Crashing Waves, Rising Beats quest (reward is Extra Class 1: Drum Master). I have to deal with 3 guys and I almost beat Jin before he charge attacks. I had to use a elixir or more to win. I forgot plenty of soloing strategies, but it would be interesting to see a rematch without elixirs.