When playing as the Black Witch you can resist spells cast on you. This option appears after the spell has been cast, not while the timer is counting down. You get to choose to try to Resist the spell, and doing so let's you roll a die. If you get a 4-6 on the die you successfully resist the spell.
The easiest way to do this is to play against another player you control("Local Player"), and choosing a good spellcaster(f.ex. the Wizard) for this role is a good idea. Just continue casting spells at the Black Witch with the Wizard, and have the Black Witch Resist them until she's successfully resisted three. Then the achievement unlocks.