V made the right mistake.

[Icedaemon]

Roy getting to land the finishing blow on Xykon still proves that Eugene was right if it was Vaarsuvius who did most of the damage (and I'm pretty sure it was stated during the Battle of Azure City that Roy and Vaarsuvius are the only members of the Order who can significantly injure Xykon). Thus, if the Order defeats Xykon for good, it basically boils down to either a Vaarsuvius-dominated victory (arcane) or a Roy-dominated victory (not arcane).

If Durkon survives until the final battle and learns to take centre stage or dies soon enough for Malack to replace him, Divine magic might play an equal or larger part to Arcane.