Just complete all of the quiddich (sorry not sure on spelling) practices and matches that appear on the club notice board. If you're not sure where that is just go to you menu, select 'where is' and you should find the quiddich hall or club its something like that. Once you've found it the first time you can access the club from your main menu before you enter Hogwarts,and this is the only way to access the final three matches that you need for the achievement. Hope this helps