I got to lvl 100 in 28 seconds.......
First you have to put as much snipers as possible in the beginning (lvl 1), then after you get the bonus, start spamming balls like Scatter Sniper Cannon and Poison. When you get to 40 balls, start upgrading, don't upgrade damage only upgrade speed. Then after you get to max speed on every ball (and max scatter amount), you spam more balls until you get to lvl 100