I just beat it, level 72 has alot of bubbles also some levels you don't have to pop all the bubbles..
Some upgrades are ice harpoon which i think is bad because you have to wait for it to unfreeze if it misses a bubble, double harpoon, and sunflower which is like a constant attack thing :D
http://www.rebubbled.com/games/bubble-struggle-2-rebubbled/