User blog comment:Neppudere/Time to lay down the LAW: Legendary Archwitch Guide/@comment-26966113-20150926144516/@comment-13020706-20150926222049

You are right. The rarity of the reward seems to be linked to the amount of damage dealt. My main account that does ~70m damage always gets at least a R card, Arcane Awakening or Light Valkyrie Sword. My other 2 starting accounts that deal under 10k damage ALWAYS end up getting commons or one of the common arcanas that can be bought with medals from the shop.