In case anyone is having trouble beating Aevelwan the Sealed, here's what I did
Party: Kuruha, Maria, Nosh, Clar (all lvl99+3)
Skills
Max out HP, DEF, MDF on all of them, give immunity to Confusion and Stun.
Kuruha: Learn every skill except the branch from Medicinal Knowledge I onward. Max out ATK.
Maria: Learn every skill except the branch from Lunatic Moon onward. I maxed out AGI and MAT, then put the leftover points into LUK.
Nosh: Learn every skill except the branch from Incite onward. Increase stats as you see fit.
Clar: Learn every skill except the branch from Pulse Wave onward. Max out MAT. Increase stats as you see fit (I maxed out LUK and put extras in MaxSP and AGI).
Equipment
Appropriate Spirit Weapon +2 for everyone
Monster God Shield +1 for everyone
Underworld Lord set +1 for Kuruha, Nosh, and Clar
Empress of Rage set +1 for Maria
2x Dark Iron Key +1 for everyone
Battle
Aevelwan goes through a set cycle of moves throughout 7 turns. This can only go on for (I think) 4 cycles before it just one-shots you. These moves (according to my abbreviated notes) are:
1. wail, mow down, crush
2. crush/gaze
3. corp. punish
4. disaster/darkness
5. mow down, crush
6. crush/gaze
7. sealed grief
A comma means that it'll use all moves listed, and a slash means that it'll pick between the moves.
Use Turn 1 to give Divinity to everyone with Kuruha's Outer Blood. Have Maria use Blood Sorcery Blade to get Aevelwan bleeding (which is a free 20k damage per turn). Make Nosh preemptively use Recover-All to get rid of the ailment given by Aevelwan's Mow Down. Finally, have Clar heal with Mana Ascension.
On Turn 2, use Kuruha's Seven Swords, Nosh's Cure-All, Clar's Grim Rose, and Maria's Crimson Moon. Aside from Maria, these will be the moves you use most.
During Turn 3, repeat the moves used in Turn 2, but use Maria's Crimson Moon Creation. Have Maria use this move whenever she safely can.
When Turn 5 comes around, make sure that Nosh uses Recover-All to get rid of the ailment ASAP, since it saps a significant amount of both HP and SP. Use Clar's Mana Ascension to heal in Nosh's stead.
On Turn 7, have everyone guard. Sealed Grief is the strongest attack in the cycle, and can easily one-shot you if you aren't careful.
With Turn 8, use Kuruha's Astral Ash to recover health before getting attacked. Use the rest of the moveset from Turn 1 since Aevelwan will go back to step 1 of the cycle. Maria can use this time to re-up on Hyperaemia by using Noble Fang.
Use Turn 9 to re-up Outer Blood, otherwise doing what you would for Turn 2 (except Maria of course, who can use Blood Sorcerer Fang, Crimson Great Wheel, or Blood Servitude/Bloodsucking Embrace).
Rinse, and repeat! Make sure Aevelwan is always bleeding and that you have as close to full health as possible at the end of every turn. You can also decide to use the Recover-All/Mana Ascension combo during other turns if too many party members are inflicted with the ailment from other moves. If anyone gets knocked-out, you can use either Kuruha's Cure-Raise or an Ultra Mana Potion to revive them (this shouldn't happen often, if at all). You can also sometimes squeeze in Nosh's Aurora Veil alongside a Mana Ascension for just a bit of padding.
All that's left is to pray that you finish before your turns run out!
I hope that this can be of some use, since beating this bastard took an ungodly amount of time and experimentation!