To gain a group of levels at once, you must be at Hogwarts. Go up to the seventh floor. Make sure you have about 1,000 Sickles. Buy as many Cards (Chocolate Frogs) as possible. Check your Folio Universita. Go to the card combo called "Extra Exp". If you have nine of each of the cards needed for that combo (except for the far left one, which is a rare) proceed to a battle with a few very easy enemies, or possibly a Boss. You will want to have all three people with you. Use Harry to use your card combo, Hermione to use Fumos Duo or Uno, and Ron to heal. Once you cannot use the card combo anymore, defeat the Boss or enemy.