Can Dive Down protect a creature against Pacifism?
Dive Down resolves first (since it's the last spell on the stack) and will give your creature Hexproof.

Then, Pacifism (an Aura, which by rule 114.1b is a targeted spell) tries to resolve, but its target has become invalid, so it will not resolve and end up in your opponent's graveyard.


  608.2b If the spell or ability specifies targets, it checks whether the targets are still legal. A target that’s no longer in the zone it was in when it was targeted is illegal. Other changes to the game state may cause a target to no longer be legal; for example, its characteristics may have changed or an effect may have changed the text of the spell. If the source of an ability has left the zone it was in, its last known information is used during this process. If all its targets, for every instance of the word “target,” are now illegal, the spell or ability doesn’t resolve. It’s removed from the stack and, if it’s a spell, put into its owner’s graveyard.


(source: Comprehensive Rules, emphasis mine)