This simple, little trick that allows you to skip the troll fight at the beginning of the Diffindo Spell Challenge. This saves a good amount of time.

As soon as you gain control after the troll cutscene, run into the corner onto the little slope where the tree is (The tree that falls once you kill the troll). Once there, face the troll and when he hits you the knockback sends you up the slope, bypassing the invisible wall and allowing you to proceed!

You are able to finish the challenge as normal, since killing the troll isn't a task.

(You can make the jump to the other side on the 1st try, but I missed it here)

The only issue with this skip is the camera, it doesn't like to cooperate sometimes and will not go behind Harry, making this trick slightly harder to do.