Diffindo skip is a very useful skip in this game as it allows you to skip basically all of the challenge bar fighting the dogs right before the spellbook. It is done by simply dodging over a cutscene trigger, once to get into the challenge and once to get out.

On your way into the challenge you will spawn in a small corridor. Simply walk to the end of this corridor and stand just a little on the doorline going out into the garden. Press the target button and proceed to dodge out into the garden (you should aim to land on the grass). Having done this successfully simply run over to the top left of the level where the tree would usually fall to find the invisible wall. Turn your back to it and flipendo overcharge to get through it. (See video for more details)

On your return you want to get as close as you can to the outer beaded line without actually touching it. Then while targeting the bush in front of you dodge inside and over the trigger. If you can't dodge in a direction and Harry just ducks then it means that your dodge can't be done so you may need to adjust yourself a little bit.

The trigger is basically from the middle of the outer stone line around the hut to the edge of the outer beaded line around the stone line (the side that touches the grass)

Diffindo Skip 1 Visual Reference:

Diffindo Skip 2 Visual Reference: