There's not that many big battles in the HP series. Here's a couple that come to mind...
Perhaps a scenario around the main gate of Hogwarts? Evil dudes at the main gate have to break in and take it over. Good dudes need to defend the entrance. For scenery you could do the entrance, a little bit of the inside of the castle, and maybe the bridge out front.
Something like this: http://www.mocpages.com/moc.php/287059
But you wouldn't need the whole thing.... just the front entrance area.
Maybe start with this set: http://www.kiddicool.com/assets/img/toy ... gwarts.jpg
Then add to it a front gate and some more areas to actually fight.
Another thought is a scenario from when the Death Eaters attacked the campsite for the quidditch world cup game. Basically setup a camping ground. Death Eaters roll in to kill everyone. Good guys fight off the Death Eaters. There are quite a few different Lego canvas tents out there you can use. Scenery would be easier than building Hogwarts entrance.