What is the working mechanic behind Shifter's Foil? Why does the feat disrupt shapeshifters. Does the feat only work with spells or does it apply to only attacks either melee or magical? Does it apply to any damaging method?
My thought is it applies to anything with an attack roll, but the flavor text could indicate it only applies to spells used by caster. So, in the first case Fireball would not disrupt but in the second case it would. Or do just it universally become a shapeshifter disruptor where any damage caused by you can disrupt a shifter?
Prerequisite: Knowledge (arcana) 5 ranks or Knowledge (nature) 5 ranks, ability to use any polymorph effect.
Benefit: A creature you deal damage to has difficulty using or maintaining polymorph effects until the end your next turn. To use a polymorph effect it must make a concentration check (DC 15 + twice the level of the effect). If you deal damage to an opponent under a polymorph effect, that opponent must succeed at a Will saving throw (DC 10 + 1/2 your character level + your Wisdom modifier) or be forced back to its original form. If you score a critical hit against such an opponent, no saving throw is allowed.
It says "a creature you deal damage to", with no qualifiers. Fireball counts. An arrow counts. Punching them in the face counts.
The feat disrupts shapeshifters because it says so. Come up with reasonable fluff to suit your needs.