Invulnerability:
At the Main menu screen enter the following code using the D-pad:
Up, Left, Down, Left, Right, Black, Black and White

How To Get Fairy Wings:
Collect Fairy Wings for your homework assignment for Professor Snape at Hogsmeade which are the woods outside Hogwarts. Use Hermione and cast Flipendo (Knockback Jinx) when the fairies come out and when you hit them they will fall to the floor. Then use the Glacius spell and freeze them before picking them up.

How To kill A Salamander:
Defeat Salamander by using the Glacius spell to put out the fire on its back and make it icy before shooting it with Flipendo (Knockback Jinx).