Operation Chief = Cron Castle "Mage/Wizard", found inside of the Castle. There are 6 spawn points.

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While there claims to be six spawn points, I have only found two on the ramparts on the path up towards the boss room, and have looked all over for the other four spawns. A bit of a pain, but just farming the two spawns should be adequate. Got it on my 8th round of farming with the 3.5% base chance and about 80% added chance from other effects/buffs.