Harry Potter and the Deathly Hallows - Part 1

This is unlocked through story progression. Immediately after unlocking A Clue to Investigate, you will have to survive for three minutes. If you don’t want to fight, or are playing on Hard, I suggest putting your back against a hole in the wall and holding for Protego. When less than one minute remains, two Dementors will appear in or around the house. Kill them, then use Protego again. When the time runs out, the fight is over and the achievement is yours.