Alright from my perspective this clearing is large. Lucius entered it on the opposite side from where Octavia is. He fired the first bolt of lightning and hit the skeleton because that small ball of lightning is intended for range. Then Asher comes at him, Lucius is still at the edge of the forest, facing the clearing. he dodges out of the way but gets cut. I don't really know where Asher is going to be so I left it vague. And then, if it does hit the ground, which is probably will, and depending where- it will explode like a bubble sending electrical shock waves in a small area. 

So yeah, I would say Lucius knows the limitations of his powers and the effect that it will have, he would not have done that if he was close to Octavia or realizes that she could get caught in the blast.