First Spell
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[...]
Alan decides to put the aspect "Monstrously Strong" on Bobby for a scene.
[...]
This sounds like a maneuver, which would be difficulty 3 against an unresisting target, I think. It would normally last "a few minutes" (one scene), and any more time you wanted it to last, you'd need to add more shifts for power (see the time chart, YS 315) So making it last the whole day would require another 6 shifts of power.
You could reduce the complexity by making the potion inflict a consequence when it wore off. For example, if you wanted it to last all day (normally a power 9 potion, but made is cause a moderate physical consequence when it wore off, then it would only require power 5 to create, since 4 shifts of power would come from the consequence.
And yes, they would only get 1 free tag on the aspect.
Does the complexity of the spell increase for tougher individuals (the
way it does for spells which are meant to "take out" a la the Heart Exploding Hex from Storm Front)?
Not if the target doesn't resist.
What are alternative approaches to accomplishing the same goal? Either
math wise, theme wise, or mechanics wise?
We could declare the potion as an effective might maneuver for how much you can lift. We'll call it power 6. So instead of using your might, you could just take a 6 whenever it came to lifting stuff. But that would only last 1 exchange "a few moments' as it's base time. If we wanted to make it last around an hour, that would be 6 steps up the time chart. If we go with moderate consequences again, that would make the final potion power 8.
Imho, magical effects should only be able to do one 'part' of a skill at the time. So, for example, you could make the Might potion that let you lift things, or a Might potion that made you a great wrestler, but not a potion that just made your might skill higher for a while.