Mental Defiance has only a single Level of Mastery. Invoking it costs only 2 points Mental Energy (zero casting time, zero cooldown time.) Spell is useful, for example, in blocking the attacks of Necromancers. It can also block Victor's spells in the mortal world (as will the Nethercoat), though it will not block Victor's Spirit's spell attacks in the Twilight World.
Mental Defiance is an active Mental Skills. It must first be placed on the game's QuickBar before it can be invoked. The effect lasts as long as the activating button is pressed, but Scarlett cannot initiate other physical or mental attacks while using Mental Defiance. She can, however, be attacked physically.
Scarlett can only learn the Mental Defiance Mental Skill after she has learned at least one Level of Mastery of its prerequisite, Echoes of Twilight; after she has accumulated sufficient Skill Points; and after she finds a Necromancy Trainer (possibly Aeris in the Inner City or Amago in the Arsenal District Catacombs).