Originally Posted by Bored
Stone Shower can't be leveled up and is TM free.
I think the guide is referring to regular level 10 (considering you should have some base DA by then)
As for the guide:
Fan of Knives has an awkward AoE pattern like siren song. It takes a lot of DA to 1hit later on in the game when no monsters have a physical weakness. Even though it has a very fast cooldown in comparison to siren, it's awkward AoE makes it hard to land two hits on the same mob. By my experience (i deleted the skill soon after getting it), siren song is better to control. Fan of knives sometimes hits every monster around you but sometimes hits just the one you target (it's not a press to cast skill like wind blade or gravity crush.)
This information may keep a fox from adding to Fan of Knives and regretting it. On the other hand, there is a new shuriken skill coming out....