if opponent uses first ability of Azorius Guildmage could I cast a Act of Aggression in response and take control of the guildmage, and then counter the Guildmage's first ability itself which was about to tap my creature?
if opponent uses first ability of Azorius Guildmage could I cast a Act of Aggression in response and take control of the guildmage, and then counter the Guildmage's first ability itself which was about to tap my creature?
Yes, that works fine.
Fluttershy is best pony.