Work both sides of the infection on this level. Do not just concentrate on one side as you will not have enough points to get a star nor will you have enough bubbles in the ceiling using only one side to complete the level with a star.
Try and take down the bubbles on the outsides and work your way in towards the infection. If you hit a bubble next to the infection it will spread so try and be as accurate as you can with your shots.
The ceiling is visible from the beginning of the level but do not try and get it down too quickly otherwise you will not have enough points to pass the level. Use the bubbles in the cauldron wisely.