Great, original idea
THe idea behind this game is a lot of fun. However, I simply went for the last level right from the get go. The objective of the game is not hard, however, the difficulty itself should come from the locational memory of the letters and numbers, as well as the ability to match the correct letter with corresponding number.
When you start making the things pulse in and out, make the bubbles different colors that blend right into the background, and stick a difficult control system in(once you click a letter, you must click a number before you can select again) in, you're taking away from the true point of the game, and just making it a strain on your eyes.
I came close to beating the game within my first five tries, and it was so addicting that I wanted to play more and beat the last level, but the colors and pulsing was so unpleasant on the eyes that I refused to continue.
If you're intent on keeping the pulsing and colors in effect, i would suggest that a further update be focused on making everything much easier on the eyes. also, maybe fix so that you can unselect a letter (right mouse button?), or go from number to letter, not just letter to number.
Other than that, a fun and addicting game.