There are YouTube videos for most levels that I use when I get stuck.

I tried going back to easier levels to get through the level rush but it didn't advance my progress with the level rush unless I completed new levels :( At level 1219 currently and it is slow moving through the levels without spending money or using boosters so I don't always get through 20 levels in the allotted time. Very frustrating that this game is being rigged to only let you win occasionally with the right color balls.