Can Player 2 play Stupefy in response to Arena Bar, let it resolve, and then play Gust on the now-reduced might unit before Player 1 can act?

Ruling: No. Once Player 1 passes priority after Stupefy is played, the chain resolves completely. Player 2 cannot play Gust targeting the reduced-might unit before the chain finishes resolving and the unit moves to the battlefield.

Sequence:

• Player 1 plays Arena Bar to buff a unit
• Player 2 responds with Stupefy
• Player 1 passes
• Stupefy resolves, reducing the unit's might
• The chain completes resolution
• Only after the chain fully resolves can new spells be played

Nuances:

• You cannot cast Gust targeting a unit with more than 3 might and then hold priority to cast Stupefy, because Gust requires a legal target (3 might or less) at the time of casting
• Legend abilities without Action or Reaction keywords cannot be used to respond to Action/Reaction speed spells