Why not make 1 event, call it "random ass rocks" or whatever. Make the event have no charset, and a parallel process every so many frames move to a random x and y coord, and play the animation on itself, and if the it's touching the player, inflict damage.
I don't know if you'd be okay with just 1 event or you really just don't want any at all, because I really don't think that this 1 event would cause much lag.