Note: This procedure involves editing a game file; create a backup copy of the file before proceeding. Use a text editor to edit the "defuser.ini" file in the "a gamesharry potter and the chamber of secretssystem" folder. Scroll down to the end of the file to find the following text:

[HGame.Harry]
bInvertBroomPitch=False
bAutoCenterCamera=True
bMoveWhileCasting=True
bAutoQuaff=True
fDamageMultiplier_Easy=1.2
fDamageMultiplier_Medium=2.0
fDamageMultiplier_Hard=3.0

You can change the values of "fDamageMultiplier_Easy", "fDamageMultiplier_Medium" or "fDamageMultiplier_Hard" (depending on the level you have selected) to 0.1 to get less damage.