Bobinator

I've had this idea for a character with rock-related powers, and I was planning for one of his powers to be to transform into a boulder and roll forward. I'm not really sure how to do this, exactly. I was thinking either. 1.) A Hand-To-Hand power with a Line of Sight custom adder, since it won't let me use that as an Advantage. Of course, I could always make a custom Advantage with that name and the same AP cost. 2.) A Charge Martial Arts ability, with the same 'Must Be On Solid Ground' Complication as the rest of the character's rock powers. Unless there's a better way to do it I'm missing. Also, could somebody please explain to me the difference between powers in a Multipower that have a fixed slot and those that don't, and why I need either?