1) Prof. Quirrell pointed out that Harry was especially vulnerable to the "Finite Incantatem" spell (to removed his transfigured armor). How does the canon mechanics of this spell work from a tactical standpoint? Is it area of effect or targeted on a per spell basis? Can a weak 1st year dispel a casting done by Headmaster Dumbledor or does caster strength play a role in its effectivity? Depending on the answers, Harry's vulnerability could be mitigated by pure strength or recursive spell depth or minor covering spells to absorb the dispelling castings. 'Just saying. :)

As I understand it, Finite Incantatem is an Area Effect spell, while Finite is targeted.

Additionally, strength definitely factors in--again, as I understand it.

edit: Strength has to factor in somehow. Even ignoring the ridiculous unbalancing effect that would have, they're in Hogwarts. If it would cancel every spell in the area, regardless of strength, Hogwarts itself would be affected with every cast.