Play the "Acid Leak" level until your ball gets smaller. While small, roll down to the place you build the bridge, but do not roll across the bridge. You can now carefully roll slowly beside the wall even when there is no platform. You must roll to the right side. There are many edges to avoid. You will eventually be exactly under the place where the "small-ball" tool is placed. Roll one field to the right then roll down on the hole and press [Action] to warp to level 18.