Solasta: Crown of the Magister

To fight Exonaika you must re-enter the beginning code O O O after reaching the treasure room in the area where you search for Hector.
You will fight a spectral dragon that has 350 health on Explorer mode, it is a hard fight, Exonaika is capable of dealing over a hundred damage to your whole team on the first turn. Long rest before the fight and use your best single target abilities and try to burst the dragon down in a turn or two. Use the anvil sword found in the treasure room as it is likely the best option for melee characters. I got lucky and dealt 102 damage in a single attack of opportunity using an Oath of Aunir/Oath of the Motherland paladin