In Hogwarts Legacy, your player joins as a 5th year and must work hard in order to catch up to the other students in their class. You are provided with the Field Guide and extra assistance from your professors to help you get caught up. These assignments are simple side quests that revolve around that professor's area of expertise.
Completing assignments will reward players with new spells that may be missed entirely should you not complete these assignments. Professor Sharp is the Potions Professor, and after completing his first assessment, you'll receive an owl with the second, which will reward you with Diffindo.
How To Complete Sharp's Second Assignment
After attending Potions class for the first time, Sharp will give you the first assignment. His first assignment must be completed first; then you'll receive an owl message informing you he has a second assignment for you. This time you'll need an Invisibility and Thundrerbrew potion to complete the assignment. Both potions can be purchased from J. Pippins Potions, with the Invisibility potion costing 500 Galleons and the Thunderbrew potion costing 1,000 Galleons.
If you wish to buy the recipe yourself and brew the potions yourself, they can both also be purchased from J. Pippin for the same price but will give you a chance to brew them in the future instead of paying the high cost. This assignment also only asks you to use the potions, allowing you to obtain them in any way possible or use ones you may already have.
The invisibility potion needs to be used and can be done out of combat, and anywhere you are if your goal is to complete the quest quickly. For the Thunderbrew potion, it must be used to damage an enemy, and wait until an enemy is found and drink the potion. For a short time, clouds will circle near the player, and lightning will hit periodically and damage anything that gets near. This will require you to allow an enemy to get close to complete the assignment but does enough damage to make it well worth the risk.
How To Use Diffindo
Casting Diffindo slashes enemies and deals a very high amount of damage. While it does have a bit of a longer cooldown, it is a worthwhile spell to use, either one-shotting weaker enemies or dealing significant damage to tankier ones. Getting the Talent that causes your basic attack to lower your cooldown on spells further makes this spell more useful and removes the one thing holding it back.
This spell can be further upgraded with a talent that allows it to hit multiple enemies simultaneously. This can be a little situational, as its rare enemies will clump together, but it is still a valuable bonus for the few times you hit multiple enemies with such a powerful spell.