With one infected ball in each upper corner, you will have a catastrophe on you unless you shoot to kill some clusters under them. Go for the sides first! The infection in the middle will be taken care of almost naturally, even if it spreads down all possible four rows.
After the middle infection is taken care of, it is all happy bubbling. You might have to try a few times, because the holes in the ceiling not always places themselves in your favor and the black bubbles are a bit in the way.
Lev 51 tips by Alex Walker: “I’d recommend the extra ceiling holes potion. The trick to me is take out the sides to prevent the disease spreading too low. The centre disease bubbles you can drop if you take out bubbles above them. The side infections you can’t drop”
Sometimes you get lucky and have the extra gaps in the ceiling just where the infected bubbles should be, that is quite cool. I have a video on me getting a board for 51 without any infections dripping down from the ceiling. I’ll update the post with that lucky video, when it is available on Youtube. :)
Here is a video by Jorge.