How to kill the goblins to get out of Gringott’s

• Strength - Dudley: Year 1 level 1. Alternatively, use the super-strength red brick found in Hogwarts.
• Dark Magic - Bellatrix (Azkaban): Year 7 part 2, while exploring Hogwarts between missions she in the air over the field near Hagrid's house. Use the pink platform with Hermione to get the pumpkincopter, then use it to fly around and get Bellatrix.
• Key - Mrs. Cole: Year 7 part 2. Use dark magic in the diner kitchen to unlock her.

Find the Defence Against the Dark Arts class room (follow the ghost chips from the Gryffindor common room). Put the cauldron together and brew the strength potion. Once you drink it you'll be able to open cupboard on the right which triggers an event that will unlock a wider range of spells.