Pic 310a: Skull bubbles on each side at the bottom of the screen and shadow bubble with them. Try to drop them first to stop shadow bubbles from spreading. Even that there are 80 bubbles in the cauldron may not be enough to complete the level.
Do not shoot the long line of the same colors bubbles under the long line of skulls (if possible) before shooting the linked bubbles otherwise will be very difficult to drop the skulls down as the infection bubbles will make one cluster with skulls. Sounds complicated so shoot linked bubbles first if you can and then drop whole cluster of infection and skulls. Now you can open the lock.
Pic 310b: Once the lock is open the bomb start to tick. (It does not matter if you have 3 holes in ceiling potion, it will ignite when chain is broken anyway. ) Be quick or use charm of disarm if you one it. Try to get as many green spiders as possible and clear the level. Youtube Channel BubbleWitchSaga, please subscribe!