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Every time another character increases their Craft, the Arcane Scion gains a spell. So play as the Arcane Scion, and play against the Dark Cultist or the Mutant, both of which have extra chances of increasing Craft. Or both. Possibly choose to control them as well, setting them as "Local Player". Then just play the game. Mutate as much as possible with the Mutant. Fight as much as possible with the Dark Cultist. Shouldn't take long until you've acquired five spells through them increasing their Craft, and you unlock the achievement.