Easy trophy, you need to use Confudo on Kreacher in Grimmauld place.
Level location : Year 5 - Chapter 3 - level 1Kreacher location : In the mirror, at the beginningSpell color : YellowIf you can't use this spell, don't worry, retry when you have a character for that.Or later with Harry.But don't forget, you can't use this when you start the game (if i remember)
Don't forget to take the studs too