Potions: Extra ceiling holes and Rainbow bubbles
Continue to clear the way above the skulls, working quickly to beat the infection. Drop the row of skulls. Pick a side to clear; whichever side has the most of the same color of bubbles would be a wise choice.
Shoot as high as you can to drop a lot of bubbles that contain the bombs. Clearing both sides will show you the ceiling
...and the rest is just history!