I just beat it about an hour ago and I have no idea how. I wanted and was prepared to do it like Aragorn. Figured it was the E33 version of Manus from Dark Souls, even down to the location. Just needed to strap in and learn the dodges of his Speed of Light attack and The Sword of Lumiere attack.

First phase was fine. Took minimal damage and he was down in three or four rounds? But I had realized I hadn't switched out the pictos I was using from trying to get Maelle to one shot his phase two and I hadn't switched my party members around like I'd wanted to experiment.   So I wrote this off as a serious attempt and just tried to do some damage with Verso, Monoco, and Lune. Took maybe 10~12% of his health off. 

My second team was still Maelle and Sciel form when I was doing the Maelle for damage, Sciel to set up double damage and pass back to Maelle combo. One I'd abandoned really and I'd leveled up since then so now Sciel was going first. I used her ability to give Maelle double damage and realized using her to pass to Maelle would be silly since Maelle is going next anyway. So I just give Simon a mark status to help Maelle do a little extra. Gets to Maelle and forgetting to switch my pictos back is biting me as I don't have enough for stendahl. I did have three gradient ranks though from my first team. So I hit gommage figuring this is an unserious attempt, might as well blow my load early anyway.

That gommage did 51 million and change and I won. I have no idea how. In the past it had done maybe 35% of his health? I was just staring at the screen for a couple seconds wondering how I accidentally killed Simon when I couldn't do it on purpose. 

Well just have 100% compleation left to go. Which mostly is grinding up to level 99, then a couple things in NG+. Simon gives a cool sword too.

The trick is to completely forget about healing. Respec your health stats into ATK/Luck/etc, Simon's health drain will keep you at 1 health most of the time anyways. Verso's Angel Eyes is a great insurance policy if you miss a parry and get 1 shot, so proc it as soon as you have 3 gradient bars - and keep Maelle and Sciel for the end after he party deletes you in Phase 2.

For Verso - I opted for the machine gun burn set up with the sword you get from that optional island. Stock him up with AP gain pictos and the pictos that adds multiple hits to the default attack; and keep stacking burns and using his skill that scales with how many ranged shots you take in a single round. The burn adds up over time (especially with Lune in the mix) and you get to burst him for 2 AP if you use the skill I mentioned.

The parries were just a git gud moment. I was stuck on him for a solid 4-5 days but once I got down his timings for the second phase it felt absolutely fucking glorious.

E33 was inspired by Sekiro, and the director is a huge Souls fan. Simon is entirely a Dark Souls/Berserk tribute; guy is aura farming infront of an eclipse with a lost arm, a Moonlight Greatsword x Dragonslayer hybrid all because of his lost love (painted Clea) - and I think the point of the fight is it's meant to thoroughly test not only your mechanical skill in the game with its parries in homage to the games that inspires E33, but also how you built your entire party, not just your main 3, due to his third phase party delete.

Again - hindsight is 20/20 and my view is probably softened by the fact that I did beat the fucker; but i'm okay with how relatively cheap he is because he's an optional, end game, super boss rather than a spike in the mid game or something.