Yeah, this is a tricky one. I didn't design the level, and actually I have yet to win it, but I think this is how it works:
Basically, when you start out that level, the emission statues start the first combos, and then things are falling like crazy. But those don't count as a combo I don't think, because you didn't start them (maybe I'm wrong on that).
At any rate, the problem is that a combo isn't scored and counted until it FINISHES, and in this level that's the challenge -- they just keep going higher and higher. So your job is to stop the combo, so that it actually scores it, which is unusual to say the least. Meanwhile, if you don't manage to stop it before 20 red blocks are cleared, you lose.
Hope that helps!