"Minimalist" achievement either checks only for "less than 3 items" or counts bought potions as items
Didn't work with Destrucion Mage on Easy, while bying only health and bonus spell potions
But did work with Orb Mage on easy after bying only 1 item and 1 potion
Edit: most likely the second one since, I just got "Duplicates" after bying 5th bonus spell potion