Simply load up the second level from Year 1 as Professor Quirrel or Professor Qurirel (Voldemort) and complete most of the level (easiest to use the Hermione provided) until you enter the bathroom. You must Wingardium Leviosa 3 times to defeat the Troll (doing so as either Professor Quirell character) to earn the Achievement.
I created this video to help:
Hope this helps.