If I use a damage spell that would kill an opponent's unit, can my opponent respond with Grand Stratagem to save the unit, or does my damage spell resolve first and kill the unit?

Ruling: The damage spell chain resolves first, killing the unit. Grand Stratagem cannot be used to save it because Stratagem is an action spell that requires focus and can only be used to create a new chain, not to respond within the existing chain.

Sequence:

• Your damage spell is cast and creates a chain
• That chain resolves, killing the unit
• Only after the chain resolves could Stratagem be used (if the player has focus) to create a new chain, but by then the unit is already dead