That's true. When it's 'each creature', Hexproof does not work. Since Hexproof only works for spells which target. So if someone targets your Hexproof creature in particular, the target spell won't work. If someone casts Judgment Day, for example, your creature is not targeted in particular, so it will die.
Source: I play a Hexproof Aura deck, I should know
I'm not sure I agree with that. Even if it says "each creature" that is still targeting creatures in a broad sense. Is there an official decision anywhere in the MTG Rules? I would rule against Wildfire affecting that particular creature with Hexproof...
You would be wrong. To start,
Wildfire doesn't target; do not use the word target when discussing the damage it does. Similar for
Anger of the Gods or
Chandra's Ignition. The effect goes through each permanent on the battlefield, and asks "is this a creature?" If yes, they do damage.
The text for Hexproof:
702.11. Hexproof
702.11a Hexproof is a static ability.
702.11b "Hexproof" on a permanent means "This permanent can't be the target of spells or abilities your opponents control."
702.11c "Hexproof" on a player means "You can't be the target of spells or abilities your opponents control."
702.11d Multiple instances of hexproof on the same permanent or player are redundant.
The important part is that it can't be targeted;
Wildfire doesn't target.