The trick to getting through the first part of Level 215 is to start clearing the outside of the diagonal skulls first. Before you do that, ADD a bubble to one side of the bottom line of skulls to lower the screen; that way you can reach around the skulls to clear the outside. Leave that bubble there until you drop the OTHER side; then drop the side with the extra bubble on the end.
On the second level I always work on one side only and take it down being careful of the bombs. Once one side is down it is easier to work on the other side as you can bank your shots off the blank side. Then I work on the doom bubbles being careful to take them down without hitting too many of the shadow bubbles as you need to try and save as many of your cauldron bubbles as possible.
It is really important to look and see what colours are coming up and plan your shots, take your time; do not rush, and try and have a move planned for each bubble.