The objective is to cast all defensive spell that you can learn in one fight: Sounds easy enough, but there are a few conditions that make this one a bit harder.
First of all, this one can't be done before you have united all 28 members of Dumbledore's Army, which progresses naturally with the story.
This could be a problem, because you can actually make almost all discoveries before this, and since these make your spells stronger, you should try to discover as little as possible (some discoveries will come naturally in missions, but those will keep us below this limit) before you know all spells.
The reason is simply that, with weaker spells, you need more of them to defeat the opponents, and this gives you more time to cast all six. I tried this first with discovery level 12 and hard difficulty, and even then the opponents were defeated before I could cast six, so rule number one: Discover as little as poosible.
So we got all spells. What do we do then?
Go to Transfiguration Courtyard and seek out people that are talking together. Attacking those results sometimes in that you get two or even three opponents and even more time to use all spells.
All spells are listed with RS-movements in the game manual, so just follow that list, and you should get this on one of your first tries.