First, at the beginning of the level, pinch the screen. This will cause it to zoom out. Pinch until it is zoomed out to the max. This will help you to better gage your accuracy.
Then, as R1 said, fire off your bird and, when appropriate, tap the screen anywhere to cause the birds to split into 3. With your first shot, take out all of the ice that you can. I recommend waiting to split the birds until the very last moment, and target them to hit the base of the ice.
Tip: With the Blue (splitting) birds, the further away your split them, the wider the area of damage. The closer you split them to your target, the more concentrated the damage will be (much more damage will be done to a small area). Keep this in mind when you are trying to hit deep on lvl 12.