The quest that needs you to cast ten times the DP skill (in my case, sorcerer, it was Tranquilizing Cloud I) can be easily completed this way:
Cast the spell, and before the casting is complete, have the target move outside the max range (25m with Tranquilizing Cloud I). The skill will not come out nor the DP will be consumed, but it will count towards the quest completion. The easiest way to do this is in a duel (preferably with someone who isn't laggy).
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