Unfortunate, but you can still pull it off if you’ve got a little patience.
Certify her (or whoever you want/can) for Thief and do an aux battle, any will do. When you’re down to the last enemy or 2, have them stand unarmed with no one else in their ranges and let them tank/dodge their attacks, healing when necessary.
Unlike weapon ranks, class mastery goes up whether you can counter or not so you can cheese the mastery for the Steal ability in however many turns it takes. Use the Knowledge Gem to speed it up and you can also do it as an adjutant if they can’t survive long enough.