Well, I've actually done Kill a Ghast with its own fireball, first mob I ever killed too. Also, I'm sure I've done the skeleton snipe kill, but without it being an official achievement, so it could have had chance to pop, I can't say for sure. I did take it out from quite a distance.
Looking for people to join me in an epic Minecraft build once again. Its in creative, so if you are interested, message me for details.