I would suggest using a bow and firing an arrow to get this achievement.
Although you can use a sword, I always found the timing a real pain, you get a little bit more leeway with the bow and arrow, but it still took many arrows before I got a Ghast on the right level to be able to return the fireball and make contact.
It took 2 fireballs to take the Ghast down, and worked best when I was slightly higher than it.
I had a potion of fire resistance and 50+ arrows to play with and couldn't make contact at all during my first try, but with autosave disabled I reloaded my previous save and got it quite quickly on the next attempt.