Wiping out armies doesn't necessarily require an attack. In fact, I'd expect maneuvers to be used for that sort of thing more often.
And regardless of what Harry needs to do, he clearly can't control Demonreach as effectively as he could with Demesne.
That aside, I promised a take on Intellectus. It's loosely based on Hick Jr's. Here goes:
INTELLECTUS [-varies]
Description: In one specific sphere, you are all-knowing. Any and all information within that sphere is available to you at all times; when you think of a question, you immediately know the answer.
Musts: You must select a sphere of knowledge when you take this Power. Your choice affects the cost of this Power. Something mostly-useless, like the geography of one island that the game occasionally visits, gives a cost of 1 Refresh. Something that's on the narrow side for a stunt, like how to hurt someone, gives a cost of 2 Refresh. Something that's on the broad side for a stunt, like human biology and anatomy, gives a cost of 3 Refresh. Something broad, like earth and stone, gives a cost of 4 Refresh. Extremely broad spheres, like everything ever written down, can give higher costs.
Skills Affected: None
Effects:
All-Knowing. When determining your knowledge or awareness of things within your sphere, you receive the following benefits. Your skills are treated as though they were at your skill cap. You always roll +4. You suffer no penalty when attempting to know or notice things that have been hidden, even if there's no mundane way for anyone to discover those things in your situation. You never need a library, and you can perform research just by asking yourself mental questions. If you don't immediately know or notice something it's just because you didn't ask yourself the right question.
Knowing Every Detail. You may make Declarations related to your sphere with your skill cap +2. These Declarations are not associated with any skill, and absolutely every fact or detail within your sphere is fair game.
I'm pretty happy with Knowing Every Detail, but I'm not thrilled with the Power as a whole. Seems too wordy, and rather clunky.
Maybe I should just make All-Knowing into difficulty-ignoring auto-success.
Opinions and suggestions would be appreciated. And if someone can do better, I encourage them to do so.