What happens when an opponent uses Challenge and then you Rebuke the targeted card? Do actions resolve simultaneously, and would the unit still take damage?

Ruling: You cannot use Rebuke in response to Challenge because Rebuke is an action and cannot be used when something else is on the chain.

Nuances:

• You could use Gust (a reaction) instead if the unit is small enough
• When using Gust, it would resolve first (most recently-added thing resolves first), and Challenge would do nothing since the challenged unit no longer exists