You have very few Bubbles in your starting cauldron. That means that you can not take out all the Bubbles below the doom skulls if you want to see the ceilling break. You need to drop the lower Bomb Cluster very fast and start banking off shots to drop the doom skull row. Keep in mind the 3 upper bombs (that you can not see while banking) and try to bank as low as possible. With a few bubbles under the doom skulls(see pic at your left hand) you have greater chances to drop the doom skull row fast enough.
Here is a video walkthrough for EI15-6.