You do if you want to invoke an aspect more than once.

Aspects are true even when they're not invoked, though. I don't think it's a problem if the pain-block only provides a mechanical bonus once per use.

What counts as a usage limitation? Does a time limit for when you can use them count?

I think just about anything can work. The actual rules are on Your Story page 148 if you'd prefer to consult them directly.

Generally transformation is done through thaumaturgy attacks.

Letting a Second Law breaker transform someone after taking them out with evocation could be cool.

I'm trying to come up with something that doesn't require you to spend a load of fate points, so aspects are out.

You don't need FP to use maneuvers.

Good idea. What kind of extra consequences (as in mild/moderate/severe, and should there be multiples)

One mild for anything you want, or two with some kind of usage limitation.

I don't think I've seen this stunt. Would you be able to either post it here or direct me to where I can find it?

Your Story page 151.

This power explicitly says you "suffer no stress from attacks based on pain." This seemed overpowered to me, since most of the damage that I understand stress to represent (bruises, minor cuts) would, in my opinion, qualify as attacks based on pain. After all, if they're not hurting, they rarely interfere with your actions. Also, what "attacks based on pain" means is fairly unclear to me. I tend to think of it as "inflicting damage that only affects you if it hurts" but I'm not confident about that.

Attacks based on pain refers to magical pain infliction, mental attacks based on torture, and the like. A punch isn't an attack based on pain even if pain is the main thing it ends up inflicting.

This would be a cool idea, but it's not what this power is narratively. This seems more like what the Winter Knight mantle allows you to do.

Not sure I see the difference.

It really specifically isn't this. Every time Harry uses it, he says that it puts him at risk, because not feeling pain is dangerous. Inhuman toughness isn't this at all.

That's mostly just narration, and maybe the occasional Compel. The Catch can cover anything else that needs covering.

Alright, here's a list of existing approaches. Off the top of my head, so probably not complete.

1. A character Aspect.
2. A temporary Aspect, created through a spell or a mundane maneuver.
3. Extra consequences from a stunt like No Pain No Gain.
4. Less harmful consequences from a stunt like Unshakable.
5. The Feel No Pain custom Power.
6. A stunt letting you use Conviction or Discipline for your physical stress track.
7. Inhuman Toughness with an appropriate Catch.

I'd allow a boot as weapon 1.

Power looks questionable to me. Evocation accuracy can very easily go to Legendary and beyond, so stressless evocation attacks are dangerous territory.

A time limit would help, but I'm really not keen on the approach as a whole. Even if you find a way around the inherent balance problems, it's pretty weird for pain immunity to provide such enormous durability. Armour 3 is Mythic Toughness level, after all.

A block opposes actions in general. If people want to do anything that the block affects, they need to beat the block's strength. It doesn't stack with defense rolls, you have to pick one or the other.

Armour is only for attacks. It reduces the stress inflicted by successful attacks, essentially reducing the weapon rating.

If an accuracy 5, weapon 5 attack is thrown at a target with a 4-shift block, a defense roll of 3, and armour 2, the attack hits by a margin of 1 and inflicts 4 stress.

Normally blocks and armour are not damaged by attacks that exceed them, but magical shields are an exception. If the 4-shift block in the above example was from an evocation, it would collapse after the attack. Armour from evocations does not have that problem, but you only get half as much of it from a given spell.

Your Story 252 is probably the page you want here.

Why would you want to represent it mechanically?

It works fine as a small backstory detail. Could maybe include it in an Aspect or something, but I don't really see any need to.