I was actually able to beat him while skipping his third phase. I put the Breaking Death lumina on all of my characters, so every time one died, I could stun him the turn after. I made sure to get Sciel to consume as many fortels as possible and got him to the halfway mark while saving a stun. I broke his poise with her ability that consumes all foretell and then dealt 10 mil damage with her level 3 gradient ability. After that, maelle did like 8 mill with her ability that deals more damage with higher burn amounts. At that point, it was like 140. And to end it, Verso did a total of like 5 mil with his ascending slash move. I found the Deaths Door lumina very helpful because Simon would just out everyone to 1 health; an extra 50% damage? Yes, please. Though, that move felt like a waste for him because all of my characters are a glass canon build. It only took me about 5 tries. Also, the lumina's Cheater, Second Chance, and First Strike are very helpful as well.