I have a world saved that will help a lot with this. It is a world originally made by Davehx for the Zombie Doctor and other achievements. I have crafted a fire resistance potion you can use, which could help you survive longer.
When you load up the world, you will be in the nether, and there will be a Ghast immediately in front of you. Just hit the fireballs back with your Diamond Sword, and if you hit it, you will get the achievement. If anything goes wrong, just re-import the world to try again.https://www.dropbox.com/s/cl0v5n8xlyocigm/Ghast.mcworld?dl=0