Winner's books include Nature, Sorcery, Crusade, Life, Death, and Chaos. At lv3400 his HP overflows into negative, but his HP bar still seems to function normally; I'm unsure what happens if you deplete it, as Akyuu shows his hp growing further negative with damage. He might actually die at his listed bestiary hp still, which displays normally. Since he's not a particularly hard boss, he only starts to seem threatening at, for example, 1.2x+ your party level. I've been listing his vulnerabilities. Debuff testing is not extensive at all.
Nature - Cold, Fire. Has a nuke.
Sorcery - No weaknesses? 80% mag buff and Space-Time Warp. Charge spell is Globe of Invulnerability.
Life - Dark, all status/debuffs. Rarely attacks.
Crusade - Mystic and debuffs. Resists Dark/Light. Deals moderate damage.
Death - Light. Has nukes.
Chaos - Fire, Wind, Nature, Cold. Resists Light/Dark/Mystic. Has a left-preference super DTH move, Doom Bow&Summon Chaos are single and all-target max hp reduction attacks.
Pretty sure he has forms other than Life vulnerable to Terror and Silence, which parties with Kogasa/Parsee/Mystia can consider. Mystia/Parsee's effects are more than long enough to ignore postgame's speed proration for an awhile, so they definitely still work, and can potentially enable final blow characters and Kogasa other than just their own nukes. Kogasa's probably stale by now, but Mystia's passives are excellent and Parsee should absolutely have competitive endgame damage.