Can you play a reaction (like Retreat) before a spell like Falling Star chooses targets, forcing the opponent to target their own units if you have no units left?

Ruling: Yes, this works for spells with "do this X times:" wording like Falling Star, but not for other battlefield attack spells like Void Seeker or Hextech Ray that choose targets immediately.

Sequence:

• When Falling Star is played, the spell itself has no targets
• Players can react with actions like Retreat before Falling Star resolves
• Upon resolution, Falling Star creates new triggered abilities on the chain (one for each "do this")
• These new abilities then choose their targets
• If you've removed your units via Retreat, the opponent must target their own units

Nuances:

• This only works with "do this X times:" spells because they create new abilities upon resolution
• Spells without this wording (like Void Seeker, Hextech Ray) choose targets immediately when played, so you cannot react before targets are chosen