I just did a run with a ~15 second boulder skip. I definitely think it can be done in under 10 seconds by getting the first one to despawn when you kill the slug (I managed to do it a few times after the run).
http://www.twitch.tv/mashystrr/c/2798375?t=3m45s
http://www.twitch.tv/mashystrr/c/2798375?t=3m45s