A pretty simple unavoidable achievement here. Directly after co-ordinating Slughorn's party, you will travel back to the burrow and after a short cutscene with Ginny, You will then have to duel Bellatrix Lestrange twice and then Fenrir Greyback once. Simply spam them with Expelliarmus and this will be a walk in the park. - if you are having any problems concerned with anything, leave a question in the comments!