I'd say the simplest solution would be to rule that it's impossible to do mental stress with evocation. Molly-style mindbending can be thaumaturgy. If you want to have Corpsetaker-style mental battles, then I'd suggest using these rules.
1. Evocation or psycomantic evothaum (Your discretion on which) can be used to initiate a mental duel, probably using a variant of the rules for a grapple.
2. In such a duel, each participant can attack with Conviction (or other skills as justifiable) to inflict mental stress (These attacks are Weapon 0, just like fists). Both parties can defend with Discipline, or other skills as justifiable.
3. The party who initiated the duel in the first place can disengage on her turn as a free action, but not if she has already made a mental attack that turn.
4. Fairies and other nonhuman creatures get a +2 bonus to mental attack and defense, to represent the extreme focus and alien nature of their minds. Scale this up to +4 or +6 for more powerful creatures (Mab, Molly, Atropos, Uriel,etc.)