I struggled about a dozen times with level 133 before I realized that there have to be a better way aiming through and right above the Doom Skull Bubbles. Then it occurred to me. By lifting all the Bubbles I get so much better aims. I have elaborated with this strategy at other levels as well, but nowhere else have it been as successful strategy as it is at level 133.
When you look at the tactical picture of level 133, it might be obvious without further explanation!? The yellow line is the bottom of the bubbles in the beginning. I have shot two yellow bubbles to the right to heighten all the bubbles. At the moment for the screen cap I have yet to receive a blue bubble before I can have a good shot at the purple lock at the right hand. (I got it and played 1 star for level 133 that time. :D)
Here is an additional video walkthrough for level 133, no charms in this one, thank you skillgaming for sharing this with us. :)