When using Challenge to compare units at different battlefields, can a hidden Back Off be played to stun the attacking unit, and does the stun prevent Challenge's damage?

Ruling: Back Off cannot be played in this scenario because hidden cards can only affect the battlefield they are hidden at. If it could be played, the stun would not prevent Challenge's damage because stun only prevents combat damage, not damage from spells or abilities.

Sequence:

• Hidden cards can only target units at the same battlefield where they are hidden
• If Back Off is hidden at battlefield 2, it cannot target Fiora at battlefield 1
• If a unit becomes stunned, it only affects the Combat Damage Step of a showdown
• Damage dealt through spells or abilities (like Challenge) is not prevented by stun

Nuances:

• Hidden cards with Reaction timing still respect the battlefield restriction for hidden cards
• Stun specifically prevents a unit from contributing its might to damage in the Combat Damage Step
• Non-combat damage from spells and abilities is unaffected by the stunned status