Regarding the discussion of taking out via Thaum, concessions, etc:
Taking out an unwilling/resisting target is hard, requiring you to do enough shifts of 'damage' that the target couldn't withstand it, even with a maxed roll and marking off all consequences. You can attempt such a thing with fewer shifts, but then the target can simply mark of the stress and take some consequences and avoid the full impact of the spell.
Taking out a willing target is easy. Taking consequences is voluntary, so you only need to exceed the target's stress boxes. Against most folk, this means you need only 3-4 shifts of effect (stress boxes+1), assuming the target can choose not to defend, thus defaulting to mediocre (0) defense result. This is basically equal to a maneuver. So turning a foe into a frog is a conflict in a spell, but turning yourself into a dog is fairly trivial, roughly equal to a maneuver even when 'fully transformative'.
I don't think that defense should ever be required, but even if you rule that the target has to 'defend' against it (to represent the body attempting to reject the transformation), the most you'd need is, say, Endurance+stress boxes+5 (about 10 for an average target) shifts of effect to guarantee the transformation for a target that chooses not to take consequences.
Note that the Laws and the Wardens might have something to say about it, even if the spells are easy to cast.