Healing Bomb is a splash weapon, so it would be a touch attack, and therefore armor/shield/natural armor AC don't matter.
I would have to overcome any of the target's touch AC bonuses:
- Dexterity, Dodge, Deflection, Insight, Luck, Profane, Sacred, Size, etc.
It makes sense to me that their size AC is 'set.'
But can they will away their Dexterity to AC for a single attack? Or would it be for the whole round?
Which other bonuses can they will away?