DUNT GET
1. Keep hail in air
2. Wait for it to grow to around 35m in diameter
3. Take aim
4. Hurl at large buildings
5. ???
6. Profit
I ended up beating a level because I was trying to make the largest hailstone I could to hurl at a skyscraper, but eventually the little ones brought it down, so I just hurled my 40m+ hail at an airplane :D