Priests + holy warrior medal?

[tofucsek]

As it is stated in the skill's description, only spells casted by the player counts, unit's spells excluded so it means you have to cast it from your spellbook. In my experience: leave a few enemy ranged units alive so they can't do too much harm and just keep spamming the required spells until you reach round 10 (use awaken dragon to cast twice in a round).