Make it type agnostic with a fixed amount of damage per layer up to 3 (say... 1/8, 1/6, 1/4)...you know, like Spikes. It would still be better than Spikes since it will hit flying/levitate Pokemon, and it could still be stacked with Spikes the way it is now. It would still be plenty good and useful, but not brokenly OP against Pokemon that happen to have severe a Rock weakness.
1: If you think your opinion is fact, then I will ignore you. 2: If you butcher the English language or abuse Netspeak, see #1.