never mind, I figured it out, thanks gamers
How are you supposed to destroy the green crystal?
Well you need to first get I think the slingshot from the one guard (done by returning a friend to him) and loading the rock on to it. Then once you reach the wizard and see the crystal you then used the loaded slingshot and will be able to destroy it.
Where do you get the rune?
Bribe the Duelist, he’ll give a weak rune.