Defend against Avada Kedavra by rapidly pressing the square button
Go to Ollivanders in Hogsmeade
Go to Ollivanders in Hogsmeade and ask him to create the Keepers Wand. After leaving the shop with the wand, Victor Lockwood will confront you with a proposal that you will refuse.
You'll be taken away by Lookwood's minions and a boss battle will begin in a new location. Since there are many enemies and you may be overwhelmed, try to defend yourself with "Protego" while manoeuvring away from their attacks. It is also effective to reduce the number of enemies with a combo of "Transformation" and "Ancient Magic Throw".
Avada Kedavra can be blocked by mashing the square button repeatedly
When you reduce the number of Victor Lockwood's minions to a certain level, he'll try to kill you with "Avada Kedavra". Mash the square button repeatedly to block it. After blocking, continue defeating the minions and depleting Victor Lockwood's HP.
Note that when you deplete his HP completely, the Quick-Time-Event for "Avada Kedavra" will occur again. If you successfully block it by mashing the square button again, the boss battle will end and the quest will be completed.