Push the medusa into the area with spikes and conveyors so the 3-step flag pushes the rock, then the bottom 3-step flag is pushed onto the spikes and lets you pass.
It's impossible if you're in the 1-space corner, you need to reach the top. To do that, you need to die without moving left, then exit from the left. If you've already reached the top, then push the rock onto the conveyor.