It's pretty simple for most of the playthrough until you get to the final stages, which turn into almost 7-minute time trials. I figured there would be a lot more emphasis on the comboing, but outside of the two challenges, it really doesn't matter at all. I was able to get every upgrade by just turning off my brain for comboing.

I checked out hard mode for a few stages, and it is almost entirely different with much harder variants. Every enemy drops like three damaging heads on death, just forcing you to go a bit slower. Maybe I'll go back to it when I get the DLC campaign.