How does the strategy of attacking a support unit in a skirmish work?
I've had a response here
From a user named: David F:


  It's invoking this rule. Bolded the part that's relevant for this.
  
  
    rulebookwrote: Front-Line Unit Combat Capability Effects When
    resolving a skirmish, if a player’s final attack strength equals or
    exceeds the final health value of the enemy, that player’s attack is
    said to have “sufficient strength,” meaning that he normally will
    destroy an enemy unit in the skirmish. However, the involved units’
    types, combat capabilities, and arrangement in the skirmish will
    affect whether and which figures are actually destroyed. The following
    factors govern this determination, and are considered during the
    “Destroy Units and Discard Cards” step of skirmish resolution.
    
    
    If a player has sufficient strength, and his front-line unit’s combat capability allows it to target the opponent’s front-line unit,
    the opposing front-line unit is destroyed. (The combat rules and
    examples on pages 26–33 assume this.)
    
    
    - If a player has sufficient strength, and his front-line unit’s combat capability is unable to target the opponent’s front-line unit,
    but is able to target one of the opposing supporting units in the same
    skirmish, such a supporting unit is destroyed instead. (If multiple
    opposing supporting units are possible targets, the player who
    controls those units selects which one is destroyed.)
    
    
    If a player has sufficient strength, and his front-line unit is unable to target any opposing unit in the skirmish, no opposing units
    are destroyed.
    
  
  
  Deliberately put a Valkyrie in front, go "oh sorry I can't hit the
  Zealot. I guess I'll hit the air unit behind it then".
  
  To counter this, use the Defensive Module from Brood War (ruled here
  that you can switch units within the same skirmish).