This is the one where you let Siegebreaker destroy 6 towers near the demon gate in Act 3. But where exactly is this gate? I tried letting him destroy the towers in the arena where you fight him, but it didn't do anything. What am I missing?
It's likely there's a bug with this. You are indeed in the right arena. Some have suggested sticking close to the siegebreaker, and dismissing your follower. Be sure to not accidently topple them yourself, too.