This happens late in the story, and this means you're close to the end! Yay!
This is the third time you'll face Scarecrow, and somewhat more difficult than the previous two.
Use your line gun when you reach a platform with a wall moving back and forth. Time it so you don't run into it when you slide.
You'll run the course as before, and run into a pack of enemies during this run.Defeat them, get past them, and reach the spotlight.
You'll be transported to another area with three spotlights. Defeat three waves of enemies. Each wave lights a spotlight, and after three waves you'll light all three spotlights.
Scarecrow is defeated, as well as another 50 gamerscore.
Story related, and cannot be missed.