Settings/Server Settings/General/Make Admin
Go to Admin Panel
Put on God mode
Click on Fly
Exit Admin Panel
Hold A and fly up high
Go back to Admin Panel
Click on Walk
Leave Admin Panel
Fall and get your achievement
It's best to do this when you're at the volcano.
You'll be pretty high up already
Fly up to get
Then drop down to get this achievement.
I go over this in my Video @ 18:10