Is there a way to beat the level even normally without getting rid of two medusas? Because you only need to get rid of two of them. But if you managed to somehow beat the level without doing that before then here's what you need:
Top rock right of horizontal splitter. That's basically it. Now you can destroy two medusas using the other rock in order to get the timing right. From there, it's all about working the splitters to get more cats facing right until you have enough to finish the level.
Edit: Actually, looking back now, I can't even remember how to beat it without cloning cats, so there must have been a way I don't remember that didn't kill the medusas. Honestly, I think the "secret" solution is easier now.