After completing one of the most enjoyable side quests in Hogwarts Legacy, you will need to return to Cassandra Mason and confront her about what she has done. This will trigger a boss battle in some close quarters. She will throw various spells at you but nothing too difficult. One thing she will do is make use of the clear white shield.
Up to this point, you will have seen various different shield types for yellow, red, and purple spell types. Since there is no white spell, what do you need to do to break this?
When Cassandra is using the white shield, you can only use two types of attacks to destroy it. Either parry an attack when possible and reflect the attack by holding the same button for the shield. She will also throw large rocks at you. If you parry this and have purchased the correct skill, you will be able to press R1/RB to quickly grab the rock and throw it back at her. This will also break the white shield, allowing you to attack her again.
Depending in how far you progressed before starting this mission (slight spoiler ahead!!)… you can also break through white shields using unforgiveable curses