This trophy may send people off in search of how to change the spells you cast during duelling but the answer is in the title!
Yes use your shield only because you probably have noticed by now that if you time hitting X perfectly it will rebound a spell coming towards you back perfectly towards the person you are duelling with giving the spell a chance to hit them and when it does it takes off some of their life.
Do it in the Students Duelling Club. You will find it will go quicker and easier there.
Mad Harry Potter fans probably knew instantly Protego was just as i described while some were looking to try change their spells like when they were going for the Draco's Disco Trophy but nope no need. Just use the rebound by pressing X making sure you do not attack by casting a spell of your own or you will mess it up. Once the opposition has lost the trophy will pop and that's all there is to it.
Hope this helps, just get the timing right and the spell caster will do the rest
Good luck!