How do you do The Misty Dell, Im really stuck on this I need some help..

I was stuck on that part for the longest as well. What I found to be helpful was going up the stairs in the abandon church like thing, and luring the enemies up there and killing them one at a time.

I agree. As soon as the level begins, put on the invisibility cloak. Then move slowly round the perimeter of the church until you find the staircase leading to the roof, being careful not to touch any of the Death Eaters, Snatchers or Inferi or else your cover is blown. Go up the steps, all the way to the top. Collect the Potterwatch password and then just stand there and wait for the enemies to come to you. Take them out one at a time using Expulso and grab all the potions they drop in the hope that they are the power up ones. Occasionally, you'll need to move to the edge of the roof and fire at enemies on the ground below, and just before the end you'll have to move down onto the ground to attack the last two or three enemies who are out of reach from the roof. It'll take a while but doing it this way is the only way I can see to do it.