While it’s straightforwardly WYSIWYG ‘what you see is what you get’, what the beginning screen doesn’t let you see right away is a third bomb/skull bubble trio straight up top. So, don’t shoot anything out of the frame until you can see what you might hit.
Extra bubbles would come in handy in Level 79 if the layout doesn’t have a lot of paired bubbles – that’s just the luck of the draw. If you have the coins to spare, use the extra bubbles potion. It can’t hurt, and will add to your total score of bubbles in the cauldrons.
Clear some room left and right for angle shots, but don’t forget to leave a one-bubble cushion around those bombs in your haste to head off the infection. That can be a nasty surprise if one goes off while you’re working diligently to beat the bug!
Here is a video walkthrough by skillgaming.