Iced blocks are those that look clear and glass like. The best level to grind these on is Level 1-8. You get 4 birds that are made for destroying ice and nearly all ice blocks to break. Make sure you tap to split the bird into three before it makes contact to increase your effectiveness. Pause and restart the level to repeat when there is not many iced blocks left. You can get 15-25 blocks destroyed every attempt of the level, netting around 100 per minute. It would take about 45 minutes to grind all 5000 iced blocks. Level 1-11 may be better, but you get less splitting birds to work with.