Shiki-Ouji
Palace boss
An exceptionally powerful shikigami. Only the most elite onmyoji are able to summon and bind it to paper. It can ward off disaster or cure illness, but its ordinary temperament is quite vicious.
Elemental affinity
No fixed elemental weakness recorded in the game files.
Battle Mechanics
1. Shiki-Ouji's initial affinity to all attributes is Normal. 2. Each time it receives damage from an attribute, Shiki-Ouji's affinity to that attribute permanently becomes Null. Each time its affinity changes, as a Side Effect, its damage taken is also permanently increased by 30%. 3. When Koropokguru is defeated, all of Shiki-Ouji's affinities are reset, and it loses all Side Effects.
Weakness is dynamic, not fixed — see the mechanic above. A companion Shadow, Koropokguru, is the key to resetting it.
Source: P5X game data files (monster table), cross-checked with in-game strings — not a community guess.