you try to beat it
you beat a sertan misson
You zap it until you beat it
beat the first level and press return to leaky cauldron.
One of the men has a shovel, so you use him to get past this level.
you dont
When you beat a Bonus Level in Gringotts, pick the Level's Lego brick up again. It will let you build your own level over that one.
you shoot the patronus charm at them.
After you have 100%, acquired Voldemort and rampaged the city, the game is essentially over.
that depends on which level you are on. If you are on the swamp, just jump on the floating pedestals and go as fast as you can so the swamp bubbles don' kill you.
There are several versions of LEGO Harry Potter. Each one has a different ending. Sites like GameFAQ and IGN have walkthroughs for the entire games for free. It is best to find a good walkthrough and get the details for the exact part that you are having trouble with.
You kill it
To complete the whole game, you have to have purchased all of the characters and items as well as complete all the levels.