I have beaten the final boss a few days ago. The battle wasn't as hard if you are around Level 130 with 100 Voile Level in all stats. With max buffs and debuffs, my main attackers can do around 50k-60k, except Yuugi who dealt around 80k-100k instead. Its attacks in the second phase has high-delay and the boss itself is extremely slow if its speed is fully debuffed. The problem is in this fight is trying to debuff its Magic because for some reason, the debuff wouldn't proc, and that's annoying. Aya and Hina also help make the final phase much more bearable.
Now onto the postgame, once again, I don't see anything different from the original version. The random battles and shadow bosses are frustrating as ever but with the helps of Hina and Aya, most of the fights are bearable. I wonder if the developers test the postgame using only these two.
I have tried to fight some shadow bosses at the challenge level. Of Course, in some battles I just overlevel and kick some party members out because I can't figure it out why else the game stops the hard mode after the main game. Honestly, most battles are challenging yet fun to fight that way.
Malignut Eater, Paralysis Wasp, Kraken, Orchid can be beaten at Level 150 with 120 Voile Level in all stats. However, I have beaten Poison Wasp and Jungle Demonic Eye at Level 170-180 with with 150 Voile Level in all stats.
And I just beat Azure Giant's Shadow at the challenge level. Here's my party.

Yuugi: Full HP build with 2 Cinderforge Sword and Grand Title can tank Giant's attacks like a boss. She takes around 8k - 10k with Giant's max debuffed normal attacks and Rasetsu Fist.
Momiji: 10k ATK and 15k Defense with Healer Subclass helps keep Yuugi in a healthy shape. Unfortunately, since Prayer of Recovery is nerfed, it only heals about 10k.
Aya: Full SPD build with 2 Costume - Ninja Lord and Grand Title. Aya has only and one duty, gives instant turn on Hina to debuff the giant like crazy.
Hina: Full MAG build. Debuff, Debuff and Debuff! She also dealt 100k with her FIre spell if the giant is fully debuffed.
Iku: Thoundcloud Stickleback, nuff said. Full DEF build.
Kaguya: Bullet Branch of Hourai, nuff said. Full MAG build.
Minoriko: Full HP build with 2 Button of Aegis. Without her MAG invested in at all, she heals about 10k.
Sanae: Full DEF build. Heals and Buffs
Kasen and Alice: They don't do much in this fight aside from being a secondary tank.
The fight went a bit messy but I lost only 1 person because RNG feels like it. The key of this battle was to have Aya uses either Divine Grandson's Advent or attack to keep Hina debuffing the Gaint's stats to -50%. Once the giant iwas fully debuffed, I would use Iku to fully buffs Yuugi, Kaguya, Kasen or Alice to attack the giant. Unfortunately, she died early due to being punched in the face so I was forced to use only Kaguya and Hina. I also had to keeps Yuugi alive at all time since no one else can survive Rasetsu nor its normal attack. Thankfully, the giant targeted Yuugi most of the fight. Its Earthquake and Great Roar, while very damaging, wasn't enough to OHKO my party members. Momiji, Minoriko and Sanae did their best to heal Yuugi at all time.
Whew, that fight was quite an ordeal. Where's the achievement for me now, game!?