Oh looks really smart UP!!!
My PCs are to face their first encounter with the mythic mechanics - Iymrith
(Wyrms of the Realms)
They are only 15th level, with other participants for and against, but our homebrew rules also likely increases their effective level by an easy 2 at the minimum.
I have also provided a few safety-nets for the combat which showcase the thematic destruction that is visited on one and their equipment when face off against such an opponent.
Reduce AC effectiveness of a shield or armour by 1 per 5 damage reduced (any item whose AC is reduced to 0 is destroyed)
Reduce damage by d12 per magic category to have a magic item Sundered (half that if magic item is a consumable)
Reduce damage by 10 to gain a level of Exhaustion
Reduce damage by 20 and roll for the character on the Lingering Injuries table