What happens when attacking units (including Fiora) cannot kill all defending stunned units in a showdown?

Ruling: If attacking units cannot kill all defending units during a showdown, any remaining attacking units are returned to their controller's base at the end of the showdown.

Sequence:

• Attacking unit (Fiora) assigns damage in combat
• Since she cannot clear the entire battlefield (kill all defenders)
• She must retreat to base at the end of the showdown