Two ways to save 12 pets in Pet Rescue Saga Level 741
The first method shown in these examples is to NOT pop the balloon.
This stops this virus from spreading at all. It does mean that you need to get the Pet carriers lined up to open them.
Then concentrate on one side carefully matching blocks so that they would stay in groups all the way down the board.
The second method USES the virus.
While working on getting the Pets down on one side allow the virus to multiply on the other to create a large group that can be cleared all at once (and drop the Pets more efficiently).
The built up block buster (hammer) booster that is within this game is great for breaking the checkered pattern that the virus spreads in.